Asriel is the final boss of the True Pacifist Route. At New Home's Barrier, Toriel, other main characters, and monsters stop Asgore's encounter. After absorbing the six human SOULs, Flowey ensnares and absorbs the monster SOULs. Flowey then transforms into his original form, Asriel Dreemurr, intending to defeat the protagonist and restart UNDERTALE from the beginning. However, he mistakes the protagonist for thefirst human.

Asriel's attacks consists of a variety of rainbow and star-shaped attacks. It includes fire, diamond bullets, rainbow-colored straight beams and lightning, and blade swipes. In his final form, his attacks are a mix of rainbow-colored comets and a powerful beam.

After enduring many of his attacks without health depletion, 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.