This achievement will unlock after you find the final item in the Trials of Fear dlc, it is located in:
and isn't unlocked until after you traverse basically the entire dlc.
What this item does:
*** Spoiler - click to reveal ***The Shell Mirror will let you dash through the eyeball enemies found in the dlc area and kill them without having to remove the stalk like you previously did with the Skin Knitter
For those who have already played this game, this area shouldn't give you any trouble but a few tips for traversing the area:
1.)Be wary of the amount of backtracking that you have to do, this area seemed to have more than alot of the other areas present in the game.
2.)There's these "pair" platforms present in this area where one platform will give you a buff and the other needs that buff to activate and open an area, for the buff to stay with you, you CANNOT GET HIT. blocking damage is OK thankfully!
3.)Having fully essence and infusion of salt will be beneficial here since it will let you tank alot of the enemies.
4.)If you feel like it: you can also activate the cheat code for infinite energy, which doesn't disable achievements!