id Software has long been popular with the Linux crowd for releasing their game engines as open source and their generous shareware packages which offer quite a bit of game play for free before you have to pay anything. But much of the focus has been on their more famous milestone games, like Doom and Quake. The two that seemed to get overlooked were Heretic and Hexen.
Heretic was a game which was based on the Doom engine and produced in joint cooperation by id and Raven Software. It has many improvements on the Doom games, including an inventory system, the ability to look up and down, a flying power-up, and lots of eye and ear candy effects. In my opinion, it's under appreciated.
I ended up grabbing the shareware DOS version and playing it in DOSbox. If you experience problems with the game's speed, hit DOSbox's keys for increasing and decreasing cycles (Ctrl-F12 and Ctrl-F11) and frameskip (Ctrl-F8 and Ctrl-F7) to tweak the performance to your system.
The play is engaging and the performance is smooth. The game looks and feels not at all like Doom and more like Quake classic. The game's world takes place in a fantasy setting instead of an industrial-science fiction setting. And the atmosphere is nicely completed by Kevin Schilder's moody Gothic music score. Also, the sprites appear to have a decent set of AI behaviors - they have a small range of offensive and defensive behaviors instead of just charging at you flailing away like Doom's monsters did.
All in all, a forgotten classic.