We've all played Tribunal, and we've all probably become experts at obtaining the King's Oath sword. Many an unwary player has picked up this sword only to wonder why they were suddenly losing health, possibly dying. Careful examination reveals that the sword is enchanted to do 20 pts for 3 seconds Fire Damage... on self.
I've Googled and searched this, and would like to posit my own theory- general consensus seems to be that this is either a tradeoff to a decent sword, or a Bethesda oversight. But what if it's not?
What if the sword isn't an oversight? Tienius Delitian explicity states that the sword is cursed to kill 'thieves and traitors'. And it's a well known fact that Helseth has tried to have you killed repeatedly, and he's an extremely clever murderer.
What if the sword is designed to take advantage of the unwary player (at least at first): they glance at the sword, see Damage Health, Paralyze, Fire Damage. "Oh, sweet," says our Nerevarine. "That's a good sword, I'll use it!" So they go into the sewers, likely on the 'kill Goblins' mission for Almalexia, and get into a fight. Confident in their abilities, the Nerevarine doesn't even notice that the sword is backfiring on them until they keel over dead (as I and probably a few players did at first). "Wait, what?" they ask. On reload, maybe it happens a few more times before the check the sword and see the enchantment.
My theory, and it's an excellent one, is that the sword is nothing more than another attempt by the Good King Hlaalu Helseth to kill you.