One of the best games ever, like doom but with role playing and some really creepy creatures to kill. It's a CLASSIC.
It's based on the interface of 3-D shooter of Doom, the one which predominated at the time, but you can pick different races with different skills, suit your best. Though in the game you go doing things and interacting with the environment by pure coincidence, it takes some time to get you to know what you're doing and what you must do to complete each level or which is the right way to press switches and what it does. It's the only complicated thing about the game. I would recommend to play it as the all sequel which involves Heretic I and II and Hexen I, II and the expansion.