Just pickpocketed his heart to check weapons. Ordinary forsworn axes, not poisoned, no poison in his inventory. Unless they automatically get poisoned when they enter combat or something doesn't seem like it could be that. Wouldn't surprise me if they did have uber scaled poisons the player couldn't access though.
Also just tested with dremora lords. He kills them incredibly fast, if he's using poison his weapons are treated as being always poisoned because he doesn't hit any less hard with subsequent swings. I calmed and healed him after he kill the first two, he killed the next two just as quickly.
If this thing isn't some sort of design oversight Bethesda are jerks to melee characters.
Great, now I'm scared. Nothing should defeat a dremora that easily. I don't mind something that gives dremoras a hard time, but these minions of Oblivion are supposed to be hard, not one-shot killed.