Metus the black necromancer was her name. So far, we had 3 Majin events: Furfur (counter), Amon (fire aura that slowly depletes your units' HP if they are around him), and Vepar (current Majin).
Isn't plat armor a commonplace thing in Aigis by now? I mean you asked before about feeding spirits and plat armor for silvers before, so I have no idea why you asked when you would use it. Unless you meant black armors, then they give 40000 exp with 3 spirits of the same rarity as unit being raised. It is the equivalent of 5x plat armors with 3 spirits each. I used them to raise a black unit at level 62 to level 80 pre-AW.