Dungeons are much harder than normal missions or bosses. They take longer but grant higher valued rewards.

To enter a dungeon, you have to meet the level requirement and need to spend a dungeon crystal of the respective area. You can get dungeon crystals from Copper and Golden Chests, Boss missions or the Guildshop.

If you can't beat the dungeon mission, booster potions from the Alchemist or a Mercenary can help you in your next try.