This is the Dungeons & Dragons stat card themed coffee mug available from Etsy seller Level1Gamers (11 or 15 ounces, $15 or $20). As you can see, the mug isn't a very useful item. *eyes blistered palms* Probably better than your bare hands for holding coffee though. Still, being so useless I almost feel bad about using my rogue's sneak attack ability to kill a fellow party member and steal it.
Thanks to Adrien, who agrees it's important to drink directly from the glass coffee maker carafe in the break room at least once to let your coworkers know you're not to be messed with.