The Ossuary is an extra episode for the Heretic: Shadow of the Serpent Riders expansion. It is quite difficult and may be compared to Thy Flesh Consumed, the single extra episode for Ultimate Doom. Like Thy Flesh Consumed, it also does not have an intermission screen between maps.
The Ossuary takes the player, Corvus, to the shattered remains of a world conquered by the Serpent Riders centuries ago. The episode, like The Stagnant Demesne, features no new monsters, weapons or graphics. There are various level themes, mainly centered on the concept of a broken and decaying world.