The Crypt of Decay is two and a half things in one name, all, unsurprisingly, related to various games.
The half of the 2.5 things is a dungeon in the Plane of Disease area of the PS2 hack-and-slash dungeon crawler Champions: Return to Arms, which is part of the EverQuest series. It acts as a simple dungeon through which one can obtain an item called the Bloodstone of Disease, which is one of a set of 11 that give the player access to a boss dungeon and the most powerful sword in the game.
The first full thing is part of the actual MMORPG EverQuest (in the Planes of Power Expansion), and is found, shockingly, in the Plane of Disease. It's purpose is to act as a farming area suitable for characters level 55-65. It can be accessed from the Plane of Tranquility from level 55 as well as by killing Grummus in the Plane of Disease. It's also known for being particularly friendly to AoE users, who can easily clear the area to farm for XP and Platinum.
Finally, the Crypt of Decay is also the third map of the second episode, Realm of Black Magic, in the original Quake. The total enemy counts for the map are (18/32/48), for (Easy/Normal/Hard). This map introduces the Rotfish (3/6/6) and Death Knight (6/10/16) enemies, as well as reintroducing the Shambler (1/2/3) for the Easy difficulty. Other enemies are the Ogre (3/7/15), the Fiend (1/1/1), and the Zombie (4/6/7). Interesting features include the fact that the vanilla Quake engine counts Rotfish as 2 enemies in its count, making the enemy count (21/38/54) to the game's perception, as well as the inclusion of the Well of Wishes easter egg area, which is home to the famous Dopefish, well known to Commander Keen fans.
"The second dumbest creature in the universe, this creature's thought patterns go "swim, swim, hungry, swim, swim, hungry." They'll eat anything alive and moving near them, though they prefer heroes."
— Commander Keen 4 Manual