Asriel Dreemurr/In Battle

After absorbing the six human SOULs and the SOULs of all monsters in the Underground, Flowey transforms into Asriel Dreemurr.

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, 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.

Attacks

 * 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.
 * 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 swipe 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 to create a powerful vortex akin to that of a black hole that attempts to draw the protagonist's SOUL into the creature's mouth (presumably devouring it) along with several damage-dealing debris (assumed to be pieces of the surrounding ground). If the protagonist is hit during this attack, their HP drops to 1, and all succeeding attacks do no damage. 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.

Strategy
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 someone 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. At the same time, each Lost Soul uses their respective attacks and SOUL Modes. 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": assumed 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, 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.

Mechanic
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 the battle continues as normal after one turn.

Quotes
Asriel Dreemurr/Im Kampf Asriel Dreemurr/Batalla Asriel Dreemurr/En Combat Asriel Dreemurr/バトル Asriel Dreemurr/W walce Азриэль Дримурр/В бою Asriel Dreemurr/戰鬥中