Whaddya know? Haight and I agree completely agree. First time for everything?
There are a couple possibilities here. It could be a desperate thrash of blood magic. However transformation into or summoning that creature seems more likely. If it's the more likely option, that could lead to a couple opposing explanations for Erin's "origin story."
Scenario #1--Cherry merged with the summoned creature or pledged loyalty to it to survive before Erin did or could intervene. This casts her as a victim to a certain extent. Like Claudia with the specter of Jakob. It has the advantage of raising sexy rescue and redemption possibilities. In this scenario, Smirking Blackshirt probably sabotaged the ritual. Could have been a joke, or blind ego, or nefarious.
Scenario #2--Cherry herself ensured the first summoning and is about to try it again. This would make her the first female character we've met in the SJI-verse who deserves to be a mindless slave. So, this scenario has sexy and satisfying punishment and comeuppance possibilities.