So this is something that has been popping up into my mind for a few days.

How does one explain a death knight that has been turned from being a DK back into a warrior or a paladin or something? I’ve seen a few roaming around as “reborn” DKs but I thought it was otherwise impossible to do.

That’s not really a thing lore-wise. Resurrection is so rare to begin with, and who knows if it could even be used on a death kinight.