The first in a series to be called "Marathon: Cry-In-Your-Stein 3D", "Thresholds of Rite" is a level built for Tempus Irae in the style of a Wolfenstein 3D level -- that is to say, no variation in floor or ceiling height (save for obstacles). It has slightly modified textures and physics, along with a new NEFX music track that plays in-game (if you have music turned on).
To get the track to run, just drag the Music folder into your Tempus Irae folder along with the map and it should run. If it doesn’t for whatever reason, say so in the comments. If nothing else, you can just listen to the music separately, loop it on a media player while you play, or simply go without.
Much thanks to Windbreaker for helping me with a lot of the technical hurdles, as well as playing through the level and offering a couple of good tips in design. You are a true comrade.