Somehow, with Jessica's device they found Bete Noire's location and decided to go to the CORE.
At the beginning, after that they figured out Bete's location, Gaster trapped her inside of the CORE's expansion, and decided to fight her.
Despite the fact that he gained more power from Sans's death and his effort, he couldn't beat Bete and Akumu due to his lack of determination and that the Bete's "Rhabdophobia" attack was used in order to take out his own weapons.
At the end of the battle, Bete speared his soul without killing him. But Jessica and Alphys interposed and neutralized Bete using the AMR.
Alphys continued with Gaster's plan (At the beginning, Alphys had to activate the Nullifier room in order to kill them) and decided to close herself with Bete in the nullifier room with a shield active.
After that, Alphys ended up with a spear in her chest and the nullifier failed to exterminate Bete and Akumu. Undyne came to rescue her, only to accidentally kill her.
|Pre-Glitchtale Battles||Chara vs Asriel ◆ The Great Human and Monster Wars ◆ Agate Lightvale vs Copper Lightvale ◆ Integrity vs Asgore ◆ Flowey vs Sans ◆|
|Season One Battles||Frisk & Chara vs Sans ◆ Gaster vs Frisk & Sans ◆ Chara vs Sans & Flowey ◆ Chara vs Omega Flowey, Sans, Frisk & Gaster ◆ Asriel vs Chara◆|
|Season Two Battles||The AMD's trap ◆ Battle on The City ◆ Gaster vs Bete Noire ◆ Undyne vs Bete Noire ◆ Frisk vs Bete Noire ◆ Asriel & Bete Noire vs Frisk, Toriel & Asgore ◆|