Gravestone mod - Graves
This mod introduces gravestones into the game that appear upon the death of a player, villager, dog, or cat. Additionally, the mod generates cemeteries in some villages and "Wither's Catacombs" - a new large structure similar to a stronghold, where you can find valuable loot, dangerous adventures, and a final boss battle with the Wither.
Important Note: Starting from Minecraft version 1.8, the mod was split into two parts - "Graves" (only gravestones) and "Extended" (all other content). This is specifically the "Graves" part.