Along with a number of other Pokemon including Solgaleo, Lunala, the Cover Legendary Pokemon of Pokemon Moon, will also be receiving a new Z-Move in Pokemon Ultra Sun & Ultra Moon. This Z-Move is said to be called Menacing Moonraze Maelstrom, and it can only be used on Lunala's Moongeist Beam Exclusive Move.
This new Z-Move and accompanying Z-Crystal are also said to work on Necrozma, but only if it is merged with Lunala in its Dawn Wings Form and knows the move of Moongeist Beam as well.