Hand of the Shadow Crown is a High Elves unique hero unit. It is only available to the Nagarythe subfaction in campaign. It was introduced with the Alith Anar Free DLC in the Resurgent Update for Total War: Warhammer II.
The Hand of the Shadow Crown is summoned in campaign via the Invocation of Morai-Heg rite. He has the ability to assassinate any enemy hero or lord, taking them out of the game for good.
The Shadow Warriors, Alith Anar’s masters of irregular warfare, are the elite warriors of the Elven race. From among their number, the Shadow King raises up the greatest warrior with the greatest grasp of strategy and tactics to be his ‘hand’, who advises him and carries out his will with a furious precision. On those occasions where Alith Anar’s force must split into two, the Hand of the Shadow Crown leads the second contingent, bringing death to the Dark Elves with the same ruthless efficiency as his unswerving liege.
You can keep multiple Hands of the Shadow Crown at once if you miss a target, so you can take out two or all three targets at once.