A man in black named Snacksworth is the only way you can acquire most Legendary Pokemon in The Indigo Disk DLC for Pokemon Scarlet and Violet. He shows up outside the entrance of Blueberry Academy and will give you treats for legendary Pokemon encounters, but the order they are given to you is random. You'll need to complete 10 BBQs to get new Treats. However, there are some Pokemon Treats that Snacksworth will keep to himself until you complete Group quests with friends online.