Context[]
After Gaster encounters Bete at the arena and manages to bring the fight outside, he, Sans, and Papyrus reunite and fight Bete with all they've got.
Battle[]
We see Gaster attacking at Bete and hits her with a magical hand. Later on, Sans uses a Gaster Blaster to damage Bete and gets blown off towards a wall. Gaster moves towards her, though he sees Sans and Papyrus. Bete moves instantaneously towards Papyrus to attack him, however, Papyrus attacks first with a huge bone before she could even get close, knocking her into a building. After Bete gets launched towards a building, Gaster uses his magic hand to attack, which knocks Bete through several buildings, and Sans uses his Gaster Blaster again. Bete gets blasted onto a wall and lands, but not before using the wall as a launcher and bounces back to the trio. So, Sans uses his telekinesis to stop her from attacking Gaster and send her back. We later see Bete attacking the buildings as to create a distraction and flee from the battle, however, while Gaster and Sans were hiding under a blaster from the falling debris, Papyrus rushes ahead and blocks Bete's path with blue bones. Bete then attacks twice to no avail, however, the third knocks him back towards the building. Gaster and Sans then attack from the sides with two gaster blasters, but while Bete was in-between the attack, hates enhances her attacking, allowing her to counter the attack but irritating her in the process.
After screaming at both of them to stop, Bete grabs Papyrus and attempts to crush him, but Sans quickly teleports behind Bete and uses a Gaster Blaster dealing damage to Bete, saving Papyrus but leaving him injured. Gaster heals Papyrus injuries while Sans moves towards the sky, summoning his special attack Gaster Blastermination to finish her (despite protests from Gaster to not do so), however, Bete uses Rhabdophobia, taking control over all of Sans' Gaster Blasters. Gaster manages to save Sans using a kindness powered hand. Though at the end of the attack, both of his hands break, and Gaster gets himself injured. As Bete is about to use the Gaster Blasters again towards Gaster and Papyrus, Sans teleports in front of them, and after summoning his final move once more, he melts down in the process. Once the attack ends, he turns around and asks them one final thing, "are you both ok?..." before turning to dust and leaving nothing behind.
Papyrus sobs over this and Gaster, who is in an extreme state of shock and sadness, stands in front of Bete and tells her to come. She feels underestimated, and moves instantaneously behind Gaster, attempting to attack him with her scythe, but Gaster stops it with his magical hands and blasts her with determination powered hands. Shortly after that, Bete uses her claws to attack Gaster from the ground, but Gaster uses his integrity blades to cut them off, and later on, he uses a patience rope to grab Bete and throw her and justice bullets to attack her thereafter.
Gaster wants nothing but to see her demise, so he uses his ultimate move after a final scream, Polychromatism. He grabs Bete and rips her in half, pierces Bete with perseverance needles, and smashes her, but he doesn't stop there as he blasts her with a determination hand, and after getting even madder, he summons a gaster blaster and all his hands filled with determination and blasts her with them. Bete, staring horrified, is forced to sit there and unable to defend herself. Yet, while he was in the middle of murdering her, we hear Papyrus scream Gaster's name in pain due to Gaster using so much of both of their magic. Gaster then stops his attacks and gives Papyrus back his magic, whilst Bete is seen dead.
Finally, Gaster says he is so tired due to all the energy he had to use and falls asleep. Papyrus rapidly grabs him and tells him he did it. whilst Bête Noire's corpse falls into the ground. as Papyrus is seen walking out with Gaster at hand while feeling a bit uneasy about her death, Hate's aura penetrates the sky and causes a drastic change to the atmosphere. Not only that, but Papyrus sees a hallucination of Gaster dying in front of him, allowing Hate to try and kill Papyrus with one swing, however, Chara comes to the arena and takes the hit, breaking Papyrus's hallucination. In the end, we see Hate and Chara's conversation where Chara ends it by showing his animosity as his eyes turn black and he prepares to fight the creature.