in the lore of Cast Shift, Frisk and Chara were great pals. but one day, Chara died, leaving Frisk a fragment of their soul. Frisk absorbed it, since they didn't have a soul at the time, and has kept it ever since. but in the drawing, they have "absorbed" 6 souls and one fragment.

you may be asking, "if humans can't absorb other human souls, how did Frisk absorb 6? let alone Chara's fragment?"

A) the souls weren't absorbed, they were... let's say "held" by Frisk. or, if you wanna get technical, they were absorbed, but not fused. like how Void Time Trio fuses for phase 4, but then unfused for phase 5. anyways, the souls were "absorbed" at the same time, allowing the "1 soul abortion limit" to be pushed to the side.

B) Frisk didn't have a soul at the time, since their soul was shattered.

now, having 6 souls + 1 fragment puts a lot of strain on Frisk's body. which is why they never use this form. unless they need to, or are forced to by the souls themselves.

btw, the 6 souls I'm talking about are the other fallen humans. like the souls Asgore has in the barrier area.

if you wanna learn more about this AU, feel free to ask me about it. i don't have much made for it yet, or a lot set in stone, but i have some stuff. so please, ask me if you wanna.