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Phase 1: Sloth to Rough.

Sans was here encountering the human in the judgement hall, as he willing to fight the very end of hell.

Phase 1.5: Reject Fate

Sans was hit by a toy knife, it deals 927 damage, but when he finally lost, Gaster heals him before he stands up again.

Phase 2: Slaughtering on Cacophony

With the power of hope, Sans is much ruthless here, he has a weapon during a bone, sometimes his durability was quite close to block it, and once it breaks, he has reach his limit.

Phase 3: Wing's of Enigmatical

Gaster possesses Sans, and brings out his followers, the damage was no effect to deal with him, the last corridor was getting dark, his soul was not unstable, and he has more potential.