In Hatred's Shadow, you will see a dark area on the wall across from a campfire. Use any weapon to shoot the darkness (an "enemy" health bar will appear when you place your reticle over the dark). A door will be revealed and you'll see a chest with two 20 sided die perched atop taunting you from within. Refer to the below image (courtesy of iBuzz7s) for the location.
Note: If you equip a Magic Missile grenade mod and throw that at The Darkness (instead of just shooting it) the door will open allowing you to open the chest inside. However, the achievement only pops when the Darkness is slain with bullets. Save your magic missiles for a subsequent trip through Hatred's Shadow. Magic Missile grenade mods are dropped by Wizards/Mages. (thanks to DustyTheHamster and TigersRule for the Magic Missiles tips). The Darkness respawns so you can go back and re-kill it as many times as you like. This is handy if you don't have a Magic Missile when you first encounter it.