The Slime Girl Ending is an Ending in Prisoners of the Elder One. It occurs after interacting with the pool in the Garden in any way other than with the Talisman.
A tendril shoots out of the pool, pulling Erik into it and engulfing him in burning liquid. When he finally surfaces, his body is digested and transformed into a slime girl, and has to live in the pool in order to survive, though she soon finds out that she prefers 'other fluids'.