From Guild Wars 2 Wiki
The Demonic Chest appears after completing Defeat the shadow behemoth event as the finale for the Secrets in the Swamp meta event. This is a boss chest and can only be opened once a day per character.
- 5 random pieces of equipment of the fine rarity or better, based on your character's level.
- 2 random gemstones, their tier based on your character's level.