Since minecraft 1.21.11 all mobs content was moved to its own mod - "Withered lands"
As you may know since minecraft 1.8 "Gravestone mod" was splitted on two parts - "Graves"(only graves) and "Extended"(all another content).
This is the second part of the mod, which consist of additional blocks(such as memorials, spawners, skull candles, and e.t.c), structures(cemeteries, undertaker house, catacombs, and e.t.c) and monsters.
Content
Structures:
Catacombs(still not updated) - huge undeground structure with wither boss at the end.
Graves(still not updated) - just a simple grave of the fallen adventurer with some loot.
Undertaker house(still not updated) - new village house.
Cemetery(still not updated) - new village structure.
Craftable Spawners(skeleton, stray, parcher, bogged, wither, zombie, husk, drowned, piglin) - they made of 5 skull candles.
Spider spawner
Piles of bones - decorative pile of bones
Piles of bones with skull - decorative pile of bones with skull
Skull crawler piles of bones - a skull crawler mobs with hides as pile of bones
Bone blocks - a block made of bones
Withered bricks - indestructible(by mobs and explosions) block
Withered grass - indestructible(by mobs and explosions) glass block
Altar - special object which can be used to enchant and disenchant items and to ressurect some mobs.
Blightwater - new toxic liquid. It melts some blocks to another one(grass -> dirt, stone -> coblestone, gravel -> sand .....) and deals damage to mobs(except slimes and undeads) and applies "Rust" debuf.
Graves(from Gravestone - Graves mod) can spawn mobs and create fog at night