Each day, a Goblin will visit you, giving you the opportunity to battle and earn rewards from them. Goblins can appear anywhere on the Adventure mode map, including areas that are not yet unlocked.
Goblins come in five rarities: Bronze, Silver, Gold, Diamond, and the Onyx King. Higher rarity Goblins are tougher opponents but offer better rewards if you succeed in defeating them.
Rewards can include Coins, Gems, Cards, Upgrade Boosts, or Card Fragments, depending on the Goblin's rarity.
If you fail to defeat the Goblin, they will escape and you will not receive any rewards.