Adventurer had a secret, they possessed her to find out what it was. Possessor took over and started absorbing her memories as well to go through them. He absorbed too many, so he was more her than himself, and then he became completely her. Apparently, when that happens, the original mind takes over again and the possessor is destroyed, and comes out somewhere as some kind of goop. You can see it coming out of her nose at the end.