Amongst many things scattered around the world map of Zamonia that you may encounter, is the Cursed Chest.
When you interact with a Cursed Chest, the reward is not granted instantly — you must earn it by surviving the encounter.
After activation, 5 monster waves will spawn around the chest, followed by a boss. Defeat the boss within 30 seconds of it spawning to claim the chest reward.
Cursed Chests come in different rarities. Higher rarities spawn less often, but offer better reward potential: Common, Rare, Epic, Legendary.

A spawned Cursed Chest in Liberty Bay. Interact with it to start the event!