Super Lucky's Tale

This can be unlocked in the first level ‘Heavy Heads’ in the Sky Castle world. Once Master Mittins has let you pass into the second area, you’ll need to head to the left side of the level once you’re able to. Go past the blue cage, past the drums with the bee in the middle and then you’ll see a what looks like a totem pillar with fire in its mouth.

Below him is a foxhole. Head over to it and burrow down - This will unlock the "Down In The Dark" achievement. Once in the new area, you’ll have to fight 3 large flower enemies. Simply let one of them flatten you and the achievement will unlock.