Heretic is the first game in the Heretic/Hexen franchise. Here begins Raven's tradition of taking id's latest game engines, improving on them, and using them in a fantasy game.
For Heretic, the Doom engine was used. The Raven team enhanced the engine with the addition of an inventory system, the ability to look up and down, and the ability to fly, along with some lesser features such as powerful wind currents. Not counting the inventory, the core gameplay that made Doom such a hit remained the same.