Hello! I don't know if someone asked it before, but anyway. What would happen, if Sans or any other monster got stuck with Chara in their abandoned mausoleum? Would Chara be able to hurt them pretty much to kill or to posses them?

Chara’s first instinct is to possess, by any means necessary. Their spirit is a strong one, but limited in its power; they could achieve much, with a strong and capable host like Sans.