In Buffy the Vampire Slayer there is a plotline that to my knowledge never gets fleshed out. Giles gets a visit from an old frenemy named Ethan Rayne. They have nicknames for each other that are supposed to be a secret. Giles is Ripper and I don’t remember what Ethan’s was. They have some shared tattoo of a symbol and there’s some sort of spell craft involved?

Anyway, Ethan comes back a few times but they never really explain the whole thing.