Necrozma is a new Legendary that has arrived in Pokemon Sun and Pokemon Moon. It is acquirable after the series of Ultra Beast quests as well as after you complete the main game and become Champion of the Alola Region.
Once the Ultra Beast missions are complete, you must find Looker again, which is relatively easy as he remains inside the motel room on Route 2 of Melemele Island. He will inform you of a refractive shape he saw zipping across the island, which he believes to be imagined.
It was not imagined however, so head to Ten Carat Hill and make your way through to the Farthest Hollow section of the area. You may want to buy a fair few Max Repels to avoid encountering those pesky wild Pokemon. In the Farthest Hollow region, continue to pop Max Rebels while roaming around in the tall grass. Assuming your lead Pokemon is below Necrozma’s level, the legendary will still appear.
Necrozma will be level 75 and have a spree of powerful moves and a unique ability that will make it a little challenging to catch. Note that it is not an Ultra Beast so stay away from the Beast Balls. Its most powerful move is Prismatic Laser, which sends out an array of powerful light beams that deals a devastating amount of damage. However a plus to this is that it needs to be recharged after use, much like Hyper Beam, giving you a chance to recover. Necrozma’s ability, known as Prism Armor, serves to reduce any super effective damage dealt to this Pokemon by a quarter. Therefore a move that will deal 4x damage will now only deal 3x damage.