Getting there
Complete the Morgan's Leap jumping puzzle, then continue to the north and west across some floating rocks to enter the Dreamdark Enclave. Once a player has entered the area, there is a gate on ground level to the southeast they can open from the inside to admit other players directly.
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Your goal is to jump down mushroom pads into the Dreamdark Enclave, grab a Splendid Chest, and then jump up another set of mushrooms to complete the achievement (and find another chest). You will be confronted with Nightmare Mortars, Bursting Brambles, and various members of the Nightmare Court. Use appropriate caution, especially on the floor of the enclave. Mortars can knock you off the ledge, so be careful where you stand when fighting them.