Mug:You’ll get 1 Mug the first time you earn 3 Stars in a Dungeon. Mugs might also appear in Chests after you clear a Dungeon. Tap the “+” symbol next to your Soul and Ring trackers to use Mugs in The Drinking Game.
The Cat's Meow, probably one of the easiest way to get rings. Each mug gives you 10 rings, and you can always multiply it with gems! (Mugs can be obtained through dungeons only)