This is seemingly normal, but highly magical plate mail. If a command word is spoken, the suit enables its wearer and all non-living items worn and carried to become ethereal.
Sources: 2e DMG, 3e DMG, d20 SRD
You can't kill me, demon. I'm under baatezu contract. If I die, they'll get me as a troop to fight on their side forever. Ha ha.
— A clever prime trying to turn the Blood War to his advantage
