Heretic is the first game in the Heretic / Hexen franchise. It uses the engine from id Software's DOOM and transplants that game's first person shooter gameplay into a fantasy setting.

Like DOOM, Heretic consists of three distinct episodes, playable in any order, the first being available as shareware. The hunt for D'Sparil begins in the City of the Damned, continues in the alternate dimension of the Hell's Maw and comes to an end in The Dome of D'Sparil. Every episode consists of nine levels, one of them a hidden one. The goal in each level is to find the exit, killing everything standing in one's way. Keys must be found and buttons pressed to advance. Enemies include gargoyles, golems, undead knights, sorcerers, ophidian beasts and more. Most enemies exist in several varieties: some have additional range attacks, others have ghost forms and are impervious to certain weapons.

Heretic goes beyond DOOM in certain aspects. The engine has been enhanced with the ability to look up and down and the ability to fly, and wind currents pushing the player in (often unwanted) directions have been added. Also new is an inventory system. Many different power-ups can be collected and then be used at the correct time. These include health flasks, invisibility and invincibility upgrades, time bombs, and torches to light dark rooms. The Morph Ovum transforms enemies into chickens for easier dispatching, the Wings of Wrath allow one to fly, and the Tome of Power gives all weapons a substantial upgrade: projectiles become more powerful or split up upon impact, the Phoenix Rod becomes a flamethrower and the Gauntlets remove life force from an enemy and transfer it to the player.