The Graveyard is the room type in Dungeon Keeper and Dungeon Keeper 2. It serves as a repository for corpses and allows Vampires to spawn.
Dungeon Keeper
Once built, 
Imps drag the bodies into the Graveyard. Once enough corpses (around 10) have decomposed in the Graveyard, a Vampire spawns, accompanied by an audio cue from the Mentor. 
Hounds urinate on corpses in Graveyards, aiding in decomposition and allowing Graveyards to more quickly accept fresh corpses.
Although they are fairly expensive, Graveyards are often worth the cost: in addition to raising the powerful and versatile 
Vampires, Graveyards help clear the dungeon of corpses, which most creatures (other than the 
Horned Reaper) find upsetting if lying anywhere other than a Graveyard.
If you want a Vampire quickly, drop ten Imps and a 
Bile Demon in a small, locked room. Have the Bile Demon use 
Poison Cloud to kill all the Imps, then have other Imps collect the corpses. Assuming you are using the "cheap Imps" trick to lower the cost of Imps to 300 each, this lets you purchase Vampires for 3000 gold each. Alternatively, if you have a large and busy enough 
Workshop, you can drop the Imps in a locked corridor with a 
Boulder Trap, then slap it to quickly kill them.
Dungeon Keeper 2
Graveyards are expensive, and are generally not worth it if you can get strong creatures such as Black Knights and Mistresses. Vampires only have slightly more health than a Mistress, and automatically die and respawn with 1 level lower in the combat pit. Vampires also have quite an expensive upkeep, and corpses are much better left becoming an army of Skeletons with no upkeep in a cheaper prison than 1 Vampire in the graveyard.
If you can afford it though, and are maxed on portal creature food count and Skeleton count; a graveyard can increase corpse processing efficiency, as enemies you don't want converted can be made into Vampires.
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