Gameplay
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. One of the most powerful pickups that can be found is the "Tome of Power" which creates a much more powerful projectile from each weapon, some of which change the look of the projectile entirely. Heretic also features an improved version of the Doom engine, sporting the ability to look up and down within constraints, as well as fly.
As with Doom, Heretic contains different cheat codes that variously give the player invulnerability, all weapons, kill everything on a particular level. However, if the player uses the "all weapons" cheat code from Doom, a message appears warning the player against cheating and takes away all of his weapons, leaving him with only a quarterstaff. If the player uses the "god mode" cheat from Doom, the game will display a message saying "Trying to cheat, eh? Now you die!" and kills the player.
Read more about this topic: Heretic (video game)