The doppelganger

An enigmatic, mysterious, shape changer employed by the Water Barons to murder the Merchant Prince Thelonius after the heroes learned that he wished to control goblin kind by becoming the goblin king.

The doppelganger succeeded in its task, killing the would be goblin king at his "coronation". Eventually, the assassin made its way back to the Old World.

Its fate is uncertain, although an oracle revealed the doppelganger could be found physically in a grave in the ground. Thelonius, after his resurrection, also hinted at the doppelganger's death.