The Shadow Beldum event is an event that you do as the story goes or after it is completed. you must collect 3 Key Card Shards, which are found in the locations below and cannot be seen by an Itemfinder, which is obtainable in the tiny village that you can access after defeating the Ancient Kingdra. Even though the Beldum is low level, it's one of the hardest to capture in the game to date.   Once you collect all 3, go to Sheridan Wetlands to find a suspicious-looking Shadowifica