One of the best semi-"total conversions" for Hexen that touches upon almost every aspect of the game. Introduces actual RPG elements with leveling, attributes, special abilities, skills, and class focuses. Adds new classes, weapons, enemies, items, accessories/artifacts, and an alchemy system.
Found on ZDoom forum. Developed by Thetis.
Visit it for more details, detailed changelog, discussion, FAQ, feedback, and updates. The mod has its own (not yet complete) Wiki page as well.
The author recommends Zandronum source port, but it works with (G)(Z)(L)Doom too.
It's strongly recommended to add the mod to the load order for via some mod launcher and/or manager (like QZDL or DoomLauncher).