Schism it's fantasy themed gameplay expansion for Doom, Hexen and Heretic, bringing to this games some modern and innovative features. It uses Brutal Doom as base, however for more than 6 years of development it become something much more than another alteration of most famous Doom mod. Imagine what if heroes of all three worlds - Earth, Cronos and Pathoris join forces in war against demons. Now this dream come true - Schism provides new playable classes, such as Paladin, Sorcerer and Heretic. Each of Hexen and Heretic classes received special abilities and new powers after slaying The Serpent Riders, as well as obtain most iconic weapons from their worlds. However, it's not power fantasy - gameplay difficulty was increased by adding new, much more dangerous enemies, as well as rebalancing existing ones. UAC Marine - native Doom class also got a bunch of new firearms, so now available arsenal will fit all possible combat situations.
Want to play Schism with Heretic instead Doom? You can do so, as well as with Hexen. Most of enemies were reworked and got new attacks, new death animations and additional effects.
Keep in mind that Heretic balance could be more difficult than in Doom, because of really scarce health pickups.