Enigma is an artifact that can be unlocked in the Sunken Tomb level. It can be found in the bottom right of the level. You have to fall to what appears to be the bottom of the world but if you hold right you will bounce up as if off a booster. When this artifact is active, random number generation will still generate use items, but they will turn into Small Enigmas; small Enigmas are passive items that reduce use item cooldown time by 5%, and stack up all the way up to 70%.
They also had a bug that causes them to fall through the first floor that they contact, often passing through the map and despawning. This has been fixed in the latest patch.