HeXen: Beyond Heretic, or as I prefer to call it, "WHERE THE F**K AM I GOING" is a game that is quite a long way ahead of it's time. I suspect this is the first game to allow you to freely come and go between levels (once you unlock them, of course), much like you can in games like Castlevania: Symphony of the Night (although slightly unlike Castlevania, it has a hub world). The gameplay is, well, imagine doom with more melee weapons, and magic instead of guns. It's kick-♥♥♥♥. It's fast-paced, difficult, murderous fun, however it is not entirely without its flaws.
I very strongly reccomend you download a mod called Zandronum, along with downloading novert, as these give proper mouse-look (well, you can't look up and down, but the game is designed with that in mind, so it's not an issue), along with letting you customise controls and increasing the resolution of the textures, unless you're nostalgic for graphics that, in all honesty, have aged poorly. Also, the level designers seemed to have a raging hard-on for levels with poor lighting.
However, in spite of all that, plus it's insanely easy to get lost, I have to reccomend this game. The gameplay is, as you can tell from the screenshots, DOOM in a fantasy setting. You run around at mach 5 hitting ♥♥♥♥ with a lightning axe or shooting fire from your hands. Buy it, or buy the Hexen + Heretic bundle. If you like video games, fun and retro ♥♥♥♥-kicking action, you won't regret it.