After you and clover (both humans) defeat Ceroba in battle and spare her:

Starlo, Martlet, and Ceroba discuss how to hide you both from asgore and dont know how to

eventually Clover realizes that the monsters are miserable in the underground and they decide to...

give up their soul willingly...

Clover: ...Its time to go.... Martlet: No... No no no no no-we'll think of a plan! Starlo: yeah dont do somethin you regret!