"Heretic is a fantasy first-person shooter video game created by Raven Software, published by id Software, and distributed by GT Interactive in 1994.
The gameplay of Heretic is heavily derived from Doom, with a level-based structure and an emphasis on finding the proper keys to progress. The weapons are also almost exact functional copies of those from Doom, with the same key bindings. Raven did however add a number of touches to Heretic that differentiated it from Doom, notably interactive environments, such as rushing water that pushes the player along, and items. In Heretic, the player can pick up many different items to use at their discretion. These items range from health potions to the "morph ovum", which transforms enemies into chickens. Heretic also features an improved version of the id Tech 1, sporting the ability to look up and down within constraints, as well as fly."
Notes:
The original version of Heretic was only available through shareware registration (i.e. mail order) and contained three episodes.
The retail version, Heretic: Shadow of the Serpent Riders, featured the original three episodes and two additional episodes: The Ossuary and The Stagnant Demesne. This version was the first official release of Heretic in Europe.
A free patch was also downloadable from Raven's website to update the original Heretic with the content found in Shadow of the Serpent Riders.