Soul Possession is a Summoner skill.
It's a spell that allows the summoner to possess a Servant Summon and directly control its body. Stats and skills become those of the Servant's. The Summoner's original body is controlled with simple commands, as if it were a regular Summon Servant.When this skill was first implemented, the idea of directly controlling a monster attracted a lot of attention. Between Adventurers and monsters, the former can do much more and due to cooldowns and casting time, players couldn't return to their original body immediately if it was in danger. Because of this, it never became anything more than a joke skill. After the Catastrophe, the range was expanded and has once again attracted attention.