This mod adds a gravestone to the game, which "spawns" at player/villager/dog/cat death. Also this mod generate cemeteries in some villages, and a "Wither's catacombs". This is a new huge structure, like stronghold, where you can find a lot of loot, dangerous adventures and a "Wither Boss" at the end.
Visit official site or minecraft forum for more information.
Since minecraft 1.8 this mod was splitted on two parts - "Graves"(only graves) and "Extended"(all another content). This is a "Grave" part.