Map 33: Betray for Doom II (secret level exclusive to Xbox) - 100% completion

From doomwiki.org:

MAP33: Betray is a secret map exclusive to the Xbox version of Doom II, included with the Xbox Collector's Editions of Doom 3 and Doom 3: Resurrection of Evil. It can be accessed from MAP02: Underhalls, and contains SS Nazis for no apparent reason. It uses the music track "Read Me While Listening To This" which is otherwise used as the game's text screen soundtrack.

Completing this level takes the player to MAP03: The Gantlet.

It was originally a 1995 level designed by Michael Bukowski, a programmer for the Doom 3 port with Vicarious Visions. A bugfixed version, based on the Xbox version, can be downloaded at ClassicDoom.com alongside the Xbox version; in both cases, the level is located on MAP01. The Xbox version is a tweaked-up version of the original 1995 level.

The Xbox 360 port of Doom II and the Doom 3: BFG Edition version also contain this level; in the latter, all SS Nazis, with the exception of one, were removed. However, it is inaccessible in both versions because the exit switch leading to the level in MAP02 is inoperable (in the Xbox 360 version, it does nothing; in the BFG Edition version, the switch causes MAP02 to be restarted).

In the Doom II 2019 re-release for the Doom Classic Unity port, the map is omitted entirely; it is not accessible in an editor or to be found in the IWAD, as the IWAD itself is based off the v1.9 version instead.

https://doomwiki.org/wiki/MAP33:_Betray_(Xbox_Doom_II)