Finished first Quake recently (EXCELLENT game, still very much playable even after nearly 3 decades, definitely 9/10), and currently playing Perdition’s Gate, it is kinda unofficial Doom 2 expansion released in 1996, and it had chance to be included in Final Doom (which are two additional official episodes for Doom 2), but ultimately it was decided against it.
Perdition’s Gate is kinda interesting as it has some unique elements and I genuinely enjoyed it even though there are probably much better Mega WADs nowadays, later maps are too short though, and it looks like they have made them only to accomplish 32 maps total like in the original game. Secret maps are also… odd… definitely not bad, but… just odd (and disappointing to some extent I guess), although I am not going to spoil them for you. Also the locations for secrets are also weird.
Interesting and odd is I think the best description for this Expansion/WAD.
In 2050, when Bethesda releases TES 7, it will be just a launcher to Creation Engine editor with half-assed models and a single empty map where modders collaborate to make the game themselves.
In 2127 with the early-access TES 8 it will be just a README file where they say that game should be ready for Christmas of 21XX, if modders won’t fuck around and finish making the damn engine.