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. This episode like, "The Stagnant Demesne," features no new monsters or weapons, nor is there an intermission screen. Instead, the "Now entering" bumper is featured on the same background as the "(Map name) finished" one, but on some versions, the "Now entering" bumper replaces the "Time" feature. There are various level themes, all centered on the concept of a broken and decaying world.
None of the music here is original. Instead, it recycles the music from episode 1, except that map 1 ("Catafalque") uses the other episode's map 6 music, and vice-versa with map 6 ("Halls of the Apostate").
Sky Color: Grey (same as in episode 1)