This is just a simple mod that randomizes ShadowCaster monsters into Heretic. Each enemy placement in Heretic has been replaced with a randomspawner. There is forty-four or so added monsters in total. I am a big fan of ShadowCaster and the Raven Software style sprites are so visually similar to Heretic that I think they fit. I tried to find other mods that used the sprites and there were only a few here and there and not the entire bestiary. There is also support for Wrath of Cronos or Brutal Heretic RPG as an option.
Shadowcaster...that one was a challenging game to me as a kid but I did manage to finish it.