Deathkings of the Dark Citadel is the only official expansion of the original Hexen game.
Continuing the story of the original game, the protagonist picks up the Chaos Sphere at the end of the game and is then transported to the realm of the dead. To escape he must travel to the citadel of and slay the Death Kings.
This add-on pack features 20 new single player levels across three new hubs and 6 new deathmatch levels from the original Hexen team. There is no new content besides the new levels (i.e. bad guys, textures etc.)