Requiem is a megawad that contains 32 new levels. Several of the designers who worked on this project previously worked on Memento Mori and Memento Mori II. It is well-known for introducing various architectural tricks that continue to be used to this day, including faked 3D bridges. It is also one of the few PWADs that are allowed to be used in Compet-N speedruns.
Simply the Most Challenging Megawad i played back then.