Choose an area of solid ground that covers a square with side length of 25 feet. All creatures in that area must succeed on a Dexterity saving throw or receive 12d8 Bludgeoning damage, be pushed out of the area, and fall prone. Those that succeed receive half damage and are pushed out of the area. That area becomes a zone of difficult terrain.
From that area, the entrance of a mausoleum from the Abyss emerges, occupying a cube in the center with side length and height of 15 feet. From the mausoleum, a fearsome undead monster emerges and appears in an unoccupied area closest to the mausoleum and ignores the difficult terrain in that zone.
This monster can be any undead creature (the DM allows), as long as its CR is no more than 10. You have total control over this creature and can communicate with it telepathically, dictating its turn, as long as you are conscious and the creature remains in the same realm as you. Until the spell ends, the creature is friendly to you and your allies
During your turn, you can spend an action to command this creature to use its reaction to unleash an unholy nova. All non-undead creatures within 20 feet of of it must succeed a Charisma saving throw or receive 12d8 Necrotic damage. Those that succeed receive half damage. Creatures that have the Plague of Undeath have disadvantage to the saving throw.
You can end this spell using an action. When this spell ends, the monster teleports back into the mausoleum, and the mausoleum descends back into the Abyss. The entire zone remains as difficult terrain of disturbed earth and rubble until cleared.
The damage from the emerging mausoleum and the unholy nova increases by 1d8 for each hit-die you were missing when you first cast this spell (after spending hit-dice on this spell). When you cast this spell, you can select up to 5 creatures you can see to be immune to the unholy nova, spending 1 hit-die for each.