New artifact Regicider (or Regicide, Usurper or Royalbane): a sword of some type that isn’t already overrepresented in artifacts that has bonuses and warns to monsters with the M2_LORD and/or M2_PRINCE flags. Maybe it would also work on demon lords, depending on if they are considered nobility, or just overlords.

On a somewhat sillier note its attributes could also extend to queen bees and also have some ability to detect royal jelly on the map (invoke effect?)