There’s been this AU thing in my head for like 2 weeks, and I finally got around to drawing some of it. Let me explain what I have so far:

Imagine that Frisk met Gaster in that weird room, but despite the door being gone once they left, they’ve been able to communicate and see Gaster since, even years after they left the Underground post True End. Frisk befriends Gaster, and he explains a theory that the reason Frisk can see him is because they have “compatible souls” or something like that. He also explains that because their souls are compatible, it’s possible that Frisk’s soul and body would be able to be a suitable vessel for Gaster’s lost soul that’s been stuck in the void outside of time and space. Frisk and Gaster start working together to bring Gaster’s soul out of the Void and back into the physical realm by reopening that old Core thing and letting Gaster’s soul combine with Frisk’s. It works, but it’s at the cost of their universe being destroyed as well as creating dimensional tears that let other creatures from the Void come through and wreak havoc in other dimensions. Frisk and Gaster now travel through these rifts to other universes in the multiverse and stop these monsters. Eventually, as they travel, they gather different familiar companions (Sans, Papyrus, etc.) that have ended up alone in their respective universe and have agreed to help them.

Frisk can fight using Gaster’s magic almost like a mahou shoujo or something by placing the Gaster mask on their face. Their abilities are like that of a combination of the skeleton brother’s attacks as well as some other abilities revolving around Gaster’s black goop. Said goop can form shields, weapons, and other things. It’s really neat.

Their eyes only glow when this magic is in use. Frisk’s eye colour returns to normal when the mask is removed.

Gaster can speak through the mask when it is on, but he can also speak through Frisk due to their combined souls. However, this puts a strain on Frisk’s body. Excessive use of Frisk’s body will sometimes cause Frisk to vomit black gunk. It tastes very, very gross.