The name of Zen's new technique is Honoikazuchi-no-kami, one of eight thunder gods born from Izanami's rotting body. Dunno if it means a mix between flame x thunder or he has seven more up his sleeve.
Btw, I read both /a/ and mangaplus version, there's a difference in wording of last panel. Here he said "...to fight alongside you...", while it's "...to fight you as an equal" in mangaplus. I think "alongside" fit the context better, can someone clarify on this?