As you play through the campaign of Pokemon Sun and Pokemon Moon you will eventually learn about the mysterious Ultra Beasts that are threatening the Alola Region and engaging in all out death matches with the Guardian Pokemon of the region. However after you complete the main game, there is ample opportunity for you to acquire all of the Ultra Beasts and add them to your expanding collection.
Once you finish the main game and become the first ever Champion of the Alola League, you will eventually end up at home once again. After a short interaction with Hau, there will be a prompt to leave the house. Once outside there is a man nearby who hands you a special item. That item is the above Enigmatic Card which sends you on another mini adventure.
Once you arrive at the little motel, found to the left of the Pokemon Centre on Route 8, there will be a familiar face and a not so familiar face – Anabel the leader of a special taskforce within the International Secret Police, as well as Looker, our enigmatic detective friend who has found his way into many of the previous games and regions. You will be invited to go on a series of five missions, each allowing you to face a new Ultra Beast.
But first you will have to face Anabel in a Pokemon battle – have your Pokemon around level 60 as all her Pokemon are above 60.
As you partake in the missions you will be given special Pokeballs, known as Beast Balls which will allow you to catch these Beasts. You will not be able to permanently knock out these creatures and if you do faint them in battle, they will still remain in the area for you to find. A word on hunting them down – they are not like the legendary Pokemon in the previous games, in regards to them being seen in the environment. Instead they are like any other normal Pokemon encounter, you find them hidden in the tall grass and in caves.
After you complete the series of five missions and have your new Ultra Beasts in your possession, you will get a message from Looker talking about a black shape he has seen flying over Melemele Island. However due to a lack of scans and confirmation from the International Police headquarters, they brush it off as simple refraction.
However it is no Ultra Beast, but the Alolan Legendary : Necrozma.