Hexen was one of my favorite first-gen ID engine games of yesteryear. A friend of mine and I played through that game over a modem connection for a month straight. It was a blast and my first introduction to multiplayer co-op outside of the same room. We played through and beat the campaign and when Death Kings of the Dark Citadel came out, we tried once again but the magic was lost.
Seems I'm not the only fan as a group of modders has been working on an expansive project for almost six years. Serpent: Resurrection is meant to continue after another mod of their creation called ShadowCaster and is expansive in design and length. There is some technical issues which may occur as explained on the download page which is a shame considering the length of time the mod has been in production. Also, it not only takes advantage of the ZDoom engine which is required to play the mod, but may require a specific version depending on future engine changes. You will require the customary copy of Hexen as well for the IWAD resources as this is a mod after all.
Having embarked on the journey it has thus far been interesting and plays more like an RPG then an FPS, which is encouraging, but makes me wish for a newer and prettier engine to experience it on.