There are ~7 (don't remember exactly) doors in there. The chest spawns in one randomly each time you die.
So, just follow the path ahead to the first path that goes left. There is a door at the end of that path. If no chest is there, kill yourself and try again. When a chest is there, a light will be on above the door. But, until you find the chest first time, you probably have no idea what it'll actually look like or exactly where.
It's random. May take 1 try to find it, may take 15. You get loot from it once a week. It's either 2 radiant shards or energy.
Jump off the rock to skip part of the map and keep adds from coming until the very end. (It's in the video)