While Heretic was an independent game developed by Raven software, it looks, feels and plays pretty much like Doom, whose engine it uses. Main difference is that it is set in a fantasy world; Raven, after all, had started out with an RPG, Black Crypt. So instead of a pistol, a wand is your first weapon; this wand uses wand crystals as ammunition. Later you switch to an ethereal bow firing ethereal arrows, and the demons look more like the traditional kind, lacking the high-tech implants. On the whole, Heretic's sequel Hexen differs more from Heretic than Heretic does from Doom.

Alex's Heretic Page (defunct) gave the following overview:

Heretic uses a modified version of the DOOM engine. It's essentially the same interface: a three dimensional, ray-traced first-person environment. Heretic has more of a "medieval" theme. so you have goblins and demons instead of aliens, and crossbows and staves rather than shotguns and chainsaws. Heretic is just as violent as DOOM, if not more so; this game is probably not suitable for children under 10 or 12. The graphics are very well done, and seem smoother than DOOM; also, the ability to look up and down has been added, as has the option to carry items other than weapons.

In 1996 an add-on to Heretic, Shadow of The Serpent Riders, was released, it was actually a re-release with two new episodes and two new secret levels (a total of 20 new maps). For those who already owned the game, this 20-level upgrade came free for download.

Links

• Heretic Manual: Tricks for multiplayer.
• The Doomsday Engine is an enhanced and extended version of Doom, Heretic, and Hexen. It was originally based on the Hexen source code but parts of it have later been completely rewritten.
• PlanetHeretic is the place for the Heretic community to call home, so they claim. It is mostly about Heretic II.