"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."
Source: Wikipedia, "Heretic_(computer_game)," available under the CC-BY-SA License.