Before the battle begins, Asriel recognizes the protagonist as his adoptive sibling, the first human. He then morphs into an all-powerful being wanting to reset the timeline and start everything over again.
The protagonist's SOUL gains the ability to "refuse" during this fight (the heart cracks but instantly joins back together, literally "re-fusing"), and Asriel cannot kill the protagonist. The ACT button also changes to "SAVE" a few turns after Asriel enters his final form.
To defeat Asriel, the protagonist calls to their friend's SOULs, which are inside Asriel. Their friends appear as Lost Souls, and the protagonist "defeats" them by reminding them of who they were during their time with the protagonist.
After the protagonist SAVEs all the Lost Souls, Asriel's compassion, love, and hope return to him, and he begins crying. He then ends the fight and breaks the barrier.
Note: All attacks, assuming no armor and LV 1, deal 8 damage.
Fire Magic: Copied straight from Toriel, Asriel uses this attack exclusively during the first phase, before "the true final battle" begins. Asriel most likely uses this as a means of taunting the protagonist by testing how easily they can be killed. This phase can be skipped by simply waiting it out: it is time-based, and ends when the boss music's introduction does.
Star Blazing: Large, star-shaped projectiles rain down from the top-right corner of the screen in rapid succession, each one detonating into expanding rings of smaller projectiles just above the Bullet Board followed by one final, larger star that detonates directly above with a much denser ring. The upgraded version, "Galacta Blazing", is virtually the same except the small stars spin counterclockwise.
Shocker Breaker: Asriel strikes the battlefield with bolts of lightning seven times, which are forewarned by [!] boxes. The base attack simply strikes in an alternating striped pattern, first small (four times), then large (three times). The upgraded version, "Shocker Breaker II", adds a series of targeted strikes after three small lightning bolts, and finishes by sweeping the battlefield left to right with big lightning bolts.
Chaos Saber: Asriel hovers directly above the protagonist and swipes the bullet board five times with summoned blades, finishing by swiping both sides, leaving slow-moving residual sparks picking up velocity and flying across the board. The upgraded version, "Chaos Slicer", is considerably more dangerous as he picks up more speed and swipes the board six times.
Chaos Buster: Asriel summons a blaster and fires nine waves of bullets telegraphed by flashing lines, and finishes with a charged beam. The attack shoots in a pattern, which starts at three lines to four lines, vice versa. Dodge by moving when he fires three lines or the beam and remain still when he shoots four lines. The upgraded version, "Chaos Blaster", removes any patterns and fires randomly, and the charged laser creates a massive starburst that covers the bullet board.
Hyper Goner: Asriel's final attack before he begins fighting at full power. The Bullet Board covers the entire screen before a large creature resembling a combination of the DT Extraction Machine and Sans's signature "Gaster Blaster" attack is summoned. It then proceeds laugh evilly, and then creates a powerful vortex that attempts to draw the protagonist's SOUL into the creature's mouth along with a series of diamond-shaped objects that deal damage on contact. If the protagonist is hit during this attack, their HP drops to 1 regardless of their equipped items, and all succeeding attacks do no damage. The attack can be avoided, though it is difficult to do so. After this, Asriel starts using his full power, initiating the final phase of the fight.
Angel of Death: Asriel using his full power. However, his only attack in this form is a barrage of energy-based comet projectiles launched from his fingertips that enter the Bullet Board from the sides and home in on the protagonist's Soul in massive waves.
After enduring many of his attacks, Asriel eventually transforms into his final form. The protagonist is now only limited to ACTing, and all of its options have been replaced with Struggle, which does nothing. After several turns of struggling, the protagonist realizes that they cannot reach their SAVE file and perhaps saving the game is impossible, but with the remainder of their power, they can save something else.
The ACT button then turns into SAVE, and the protagonist must set out to save their friends, Undyne, Alphys, Papyrus, Sans, Toriel, and Asgore, who appear as Lost Souls with Papyrus and Sans as well as Toriel and Asgore being fought together. Individual Lost Souls are saved after three turns of ACTing while paired Lost SOULs are saved after four. It does not matter which ACT options are used, or which Lost Soul is ACTed on for the pairs. At the same time, each Lost Soul uses their respective attacks and SOUL Modes, save for Alphys (since she is never fought) who uses Mettaton EX's attacks. Saving a character fully heals the protagonist, as well as trying to save a person that has already been saved. Just like Asriel, trying to FIGHT the Lost Souls results in a MISS.
After saving all six, the SAVE option has "Someone else": implied to be Asriel. After selecting the name, the protagonist sees a slideshow of Asriel finding the first human, ending in a photograph of the first human, Toriel, Asgore, and Asriel. Trying to refuse the protagonist's advances, Asriel first launches a massive rapid-fire wave of energy bullets, but his second wave shoots fewer and less damaging bullets (which cannot hit the protagonist if they stay in the center). His attacks then turn into basic fire magic that deliberately avoids the protagonist's soul, much like Toriel's. He eventually launches an unavoidable beam that destroys the FIGHT, ITEM, and MERCY options, but the protagonist still survives with 0.000000001 HP (whether this is due to the protagonist's immense determination or simply Asriel being unable to bring himself to kill them at this point is unclear). After this attack, he loses the will to fight entirely, with the rest of the battle consisting entirely of dialogue between the protagonist and Asriel as his form fades, leaving only his head.
During the first phase, the protagonist is given two options in ACT:
Hope: Heals 1 HP, and reduces damage taken by 1 for the next turn.
Dream: Heals 4 HP, and fills the inventory with Last Dreams, which heals 17 HP when consumed.
It is impossible to damage Asriel; any attempt to FIGHT results in a MISS. Asriel cannot be spared using the MERCY option. It is also impossible to receive a game over in this fight; if the protagonist's HP reaches 0, their soul splits in half before repairing itself. The first time this occurs, text appears reading "But it refused." The protagonist then receives full health and returns to the the battle. After surviving three consecutive rounds, the dialogue resumes to advance the stages of the fight.
Asgore: Human.... It was nice to meet you. Goodbye.
(Fireball hits Asgore. Toriel enters.)
Toriel: What a miserable creature, torturing such a poor, innocent youth... Do not be afraid, my child. It is I, TORIEL, your friend and guardian. At first, I thought I would let you make your journey alone... But I could not stop worrying about you. Your adventure must have been so treacherous. ... and ultimately, it would burden you with a horrible choice. To leave this place, you would have to take the life of another person. You would have to defeat ASGORE. However... I realized... I cannot allow that. It is not right to sacrifice someone simply to let someone leave here. Is that not what I have been trying to prevent this whole time? So, for now, let us suspend this battle. As terrible as ASGORE is... He deserves mercy, too.
Asgore: Tori... You came back...!
Toriel: Do not "Tori" me, Dreemurr! You pathetic whelp. If you really wanted to free our kind... You could have gone through the barrier after you got ONE SOUL... ... taken six SOULs from the humans, then come back and freed everyone peacefully. But instead, you made everyone live in despair... Because you would rather wait here, meekly hoping another human never comes.
Asgore: ... ... Tori... You're right... I am a miserable creature... ... but, do you think we can at least be friends again?
Toriel: (sigh.) NO, Asgore.
Undyne: Ngahhhhhh!!! ASGORE! Human!! Nobody fight each other!!! Everyone's gonna make friends, or else I'll...!! I'll... ...
Toriel: Hello. I am TORIEL. Are you the human's friend? It is nice to meet you.
Undyne: Uh, yeah...? Nice to meet you! Hey ASGORE, is that your ex? Jeez. That's rough, buddy.
Alphys: H-hey! Nobody hurt each other!! ... ...
Toriel: Oh! Are you another friend? I am TORIEL. Hello!
Alphys: Uh, h-h-hi! ... (THERE'S TWO OF THEM???)
Papyrus: HEY! NOBODY FIGHT ANYONE! IF ANYONE FIGHTS ANYONE...! THEN I'LL!!! BE FORCED!!! TO ASK UNDYNE FOR HELP!!!
Papyrus: OH! HELLO, YOUR MAJESTY! PSST! HEY, HUMAN... DID ASGORE SHAVE...? AND... CLONE HIMSELF????
Sans: hey guys... what's up?
Toriel: That voice...!! Hello. I think we may... Know each other?
Sans: oh hey... i recognize your voice, too.
Toriel: I am TORIEL. So nice to meet you.
Sans: the name's sans. and, uh, same.
Toriel: Oh! Wait, then...! This must be your brother, Papyrus! Greetings, Papyrus! It is so nice to finally meet you! Your brother has told me so much about you.
Papyrus: WOWIE... I CAN'T BELIEVE ASGORE'S CLONE KNOWS WHO I AM!!! THIS IS THE BEST DAY OF MY LIFE!!!!!!!
Toriel: Hey, Papyrus... What does a skeleton tile his roof with?
Papyrus: HMMM... SNOW-PROOF ROOF TILES???
Toriel: No, silly! A skeleton tiles his roof with... SHIN-gles!!!
Papyrus: I CHANGED MY MIND!!! THIS IS THE WORST DAY OF MY LIFE!!!
Undyne: Come on, ASGORE! It's gonna be OK! There are plenty of fish in the sea...
Alphys: Y-yeah, ASGORE!! Undyne's totally right about that fish thing! S-sometimes you've just got to, uh... Stop going after furry boss monsters and, uh... J-just get to know a really cute fish...? ... It's a metaphor.
Mettaton: OH MY GOD! WILL YOU TWO JUST SMOOCH ALREADY!? THE AUDIENCE IS DYING FOR SOME ROMANTIC ACTION!!!
Undyne: HEY, SHUT UP!!! Man, the nerve of that guy! Right, Alphys!?? ... Uh, Alphys?
Alphys: ... No. He's right. LET'S DO IT.
Undyne: ?????????????? Well??? Uh??? I guess??? If you want to??? Then???? Don't hold anything back!!!
(Alphys and Undyne proceed to kiss, but Toriel intervenes)
Toriel: W-wait! Not in front of the human!
Alphys: Uhh, right! S-sorry, I got a little carried away there.
Toriel: Hee hee hee. My child, it seems as if you must stay here for a while. But looking at all the great friends you have made... I think... I think you will be happy here.
Alphys: H-hey, that reminds me. Papyrus... YOU called everyone here, right? Well, besides, uh, her. Uh, anyway... If I got here before you... How did you know how to call everybody?
Papyrus: LET'S JUST SAY... A TINY FLOWER HELPED ME.
Alphys: A tiny... flower?
(Flowey attacks the cast and entangles them)
Flowey: You IDIOTS. While you guys were having your little pow-wow... I took the human SOULS! And now, not only are THOSE under my power... But all of your FRIENDS' SOULS are gonna be mine, too! Hee hee hee. And you know what the best part is? It's all your fault. It's all because you MADE THEM love you. All the time you spent listening to them... Encouraging them... Caring about them... Without that, they wouldn't have come here. And now, with their souls and the humans' together... I will achieve my REAL FORM. Hee hee... Huh? WHY am I still doing this? Don't you get it? This is all just a GAME. If you leave the underground satisfied, you'll "win" the game. If you "win," you won't want to "play" with me anymore. And what would I do then? But this game between us will NEVER end. I'll hold victory in front of you, just within your reach... And then tear it away just before you grasp it. Over, and over, and over... Hee hee hee. Listen. If you DO defeat me, I'll give you your "happy ending." I'll bring your friends back. I'll destroy the barrier. Everyone will finally be satisfied. But that WON'T happen. You...! I'll keep you here no matter what!
(Surrounds the protagonist's SOUL with friendliness pellets)
Flowey: Even if it means killing you 1,000,000 times!!!
(Attacks the protagonist repeatedly until Toriel intervenes)
Toriel: Do not be afraid my child... No matter what happens... We will always be there to protect you!
(Flowey attacks again, but Papyrus and Undyne intervene)
Papyrus: THAT'S RIGHT, HUMAN! YOU CAN WIN!! JUST DO WHAT I, WOULD DO... BELIEVE IN YOU!!!
Undyne: Hey! Human! If you got past ME, you can do ANYTHING! So don't worry! We're with you all the way!
Sans: huh? you haven't beaten this guy yet? come on, this weirdo's got nothin' on you.
(Flowey attacks again, but Alphys and Asgore intervene)
Alphys: Technically, it's impossible for you to beat him... B-but... Somehow, I know you can do it!!
Asgore: Human, for the future of humans and monsters...! You have to stay determined...!
Flowey: Urrrgh... NO! Unbelievable!! This can't be happening! You... You...!
Flowey: I can't believe you're all so STUPID.
(Flowey absorbs the monsters' SOULs and whites out the screen.)
Flowey: ALL OF YOUR SOULS ARE MINE!!!
(Scene with a transformed Flowey)
Finally. I was so tired of being a flower.
Howdy! <Name>, are you there? It's me, your best friend.
You know... I don't care about destroying this world anymore.(After Star Blazing – Before Shocker Breaker)
After I defeat you and gain total control over the timeline... I just want to reset everything.(Before Chaos Saber)
All your progress... Everyone's memories. I'll bring them all back to zero!(Before Shocker Breaker)
Then we can do everything ALL over again.(Before Star Blazing)
And you know what the best part of all this is? You'll DO it.(Before Chaos Buster)
And then you'll lose to me again.(Before Chaos Saber)
And again.(Before Chaos Buster)
And again!!!(Before Shocker Breaker II)
Because you want a "happy ending."(Before Galacta Blazing)
Because you "love your friends."(Before Chaos Slicer)
Because you "never give up."(Before Chaos Blaster)
Isn't that delicious? Your "determination." The power that let you get this far... It's gonna be your downfall!(Before Shocker Breaker II)
Now, ENOUGH messing around! It's time to purge this timeline once and for all!(Before Hyper Goner)
... even after that attack, you're still standing in my way...? Wow... you really ARE something special. But don't get cocky. Up until now, I've only been using a fraction of my REAL power! Let's see what good your DETERMINATION is against THIS!!(After Hyper Goner – Before Angel of Death)
(Angel of Death)
Urah ha ha... Behold my TRUE power!
I can feel it... Every time you die, your grip on this world slips away. Every time you die, your friends forget you a little more. Your life will end here, in a world where no one remembers you...
Still you're hanging on...? That's fine. In a few moments, you'll forget everything, too. That attitude will serve you well in your next life!
Ura ha ha... Still!? Come on... Show me what good your DETERMINATION is now!(The protagonist now saves their friends.)
Huh? What are you doing...?(When saving someone else)
Wh... what did you do...?
What's this feeling...? What's happening to me?
No! NO! I don't need ANYONE!
STOP IT! Get away from me! Do you hear me!? I'll tear you apart!
... ... <Name>... Do you know why I'm doing this...?(Before faltering with fire magic)
Why I keep fighting to keep you around...?
I'm doing this...
Because you're special, <Name>.
You're the only one that understands me.
You're the only one who's any fun to play with anymore.
... No... That's not JUST it. I... I... I'm doing this because I care about you, <Name>!
I care about you more than anybody else!
I'm not ready for this to end. I'm not ready for you to leave.
I'm not ready to say goodbye to someone like you again...
So, please... STOP doing this...
AND JUST LET ME WIN!!!(Before Hyper beam (unofficial name))
STOP IT NOW!!!
I'm so alone, <Name>...
I'm so afraid, <Name>...
I'm so sorry.
I always was a crybaby, wasn't I, <Name>
... I know.
You're not actually <Name>, are you?
<Name>'s been gone for a long time.
What IS your name?
A nice name.
I haven't felt like this for a very long time.
As a flower, I was soulless.
I lacked the power to love other people.
However, with everyone's souls inside me...
I not only have my own compassion back...
But I can feel every other monster's as well.
They all care about each other so much.
And... they care about you too, Frisk.
I wish I could tell you how everyone feels about you.
Papyrus... Sans... Undyne... Alphys...
Monsters are weird.
Even though they barely know you...
It feels like they all really love you.
Frisk... I... I understand if you can't forgive me.
I understand if you hate me.
I acted so strange and horrible.
I hurt you.
I hurt so many people.
Friends, family, bystanders...
There's no excuse for what I've done.
(If not forgiven)
I just hope that...
I can make up for it a little right now.
... Frisk, come on.
You're gonna make me cry again.
... besides, even if you do forgive me...
I can't keep these souls inside of me.
The least I can do is return them.
There's something I have to do.
Right now, I can feel everyone's hearts beating as one.
They're all burning with the same desire.
With everyone's power...
With everyone's determination...
It's time for monsters...
To finally go free.
(Final Power plays as Asriel uses the SOULs to destroy the barrier)
I have to go now.
Without the power of everyone's souls...
I can't keep maintaining this form.
In a little while...
I'll turn back into a flower.
I'll stop being "myself."
I'll stop being able to feel love again.
It's best if you just forget about me, OK?
Just go be with the people who love you.
(If Frisk comforts Asriel)
I don't want to let go...
You're going to do a great job, OK?
No matter what you do.
Everyone will be there for you, okay?
My time is running out.
By the way...
... take care of Mom and Dad for me, OK?
Legendary being made of every SOUL in the underground.[Check before "True final battle"]
You think about why you're here now... You can feel the empty space in your inventory getting smaller and smaller![Dream]
Your items fill up with dreams.[Dream #2+]
You held on to your hopes... You reduced how much DAMAGE you'll take this turn![Hope]
You kept holding on. DAMAGE reduced![Hope #2+]
ASRIEL blocks the way![After transformation]
The whole world is ending.[Neutral]
Can't move your body.[Struggle #1-4]
Can't move your body. Nothing happened. You struggle... Nothing happened. You tried to reach your SAVE file. Nothing happened. You tried again to reach your SAVE file. Nothing happened. Seems SAVING the game really is impossible. ... But... Maybe, with what little power you have... You can SAVE something else.[Struggle #5]
SAVING over ASRIEL's file seems like the only way to defeat him..." "But having never SAVED before, you lack the power to do it.[Struggle #5 if the protagonist has no save file]
!?!?[After ACT button turns to SAVE button]
You reached out to ASRIEL's SOUL and called out to your friends. They're in there somewhere, aren't they? ... Within the depths of ASRIEL's SOUL, something's resonating..![Save]
Within the depths of ASRIEL's SOUL, something's resonating..![Save again]
You feel something faintly/slightly/strongly resonating within ASRIEL.[After Saving]
You call out to your friends with all your heart. From somewhere, you felt their support...[Saving someone who has already been saved]
You feel your friends' SOULs resonating within ASRIEL![After everyone has been saved]
Strangely, as your friends remembered you... Something else began resonating within the SOUL, stronger and stronger. It seems that there's still one last person that needs to be saved. But who...? ... Suddenly, you realize. You reach out and call their name.[Someone else]