Risk of Rain Returns: How to Unlock Commando’s Strange Prism

Firstly, choose COMMANDO (to obtain a Prism skin for a character, you have to take the exact character for which you want to acquire it).
Secondly, you have to spawn at Desolate Forest (it’s one of THE FIRST locations to spawn in, there is also a “Dried Lake” variants). It’s an easy task, because you just can reroll the location (just quit to the Title Screen and press “Start” again in the Character select)

Desolate Forest #3 is the one you need to spawn in.
If you do not have a Log Book of this location or you are not familiar, just run to the left side of the map every time you spawn in Desolate Forest, and if you are running to a place that looks like this:


Then, it is where you have to go!

Is it the right location? Moving to the next step…

Follow my lead through the screenshots:

All you have to do is to stand right on top of the blue roots and hold your “down” key to climb down the roots in the ground.
If you find yourself in the big hole shaped like an egg, do not move from the previous location (the root), just hold down yet again and you will fall into the small room with the Strange Prism near the right corner.
Grab it, and you will immediately get the trophy in-game, the Steam achievement and the skin for Commando.

Some screenshots of what it looks like in-game: