Bloom - The Watcher

Incorporeal Movement. The Bone Monstrosity can move through other creatures and objects as if they were difficult terrain. It takes 10 (3d6) force damage if it ends its turn inside an object.

Undead Pierce. If damage reduces the Bone Monstrosity to 0 hit points, it explodes and impales all those within 60ft with bone shards. Creatures within this area must make a dexterity saving throw of 18 at 60ft take 2d10 piercing damage, for every 5ft closer to bloom the creature is a further 1d10 damage is taken.

Multiattack. The bone monstrosity makes two bone crush attacks.

Bone Crush. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 35 (4d10 + 8) bludgeoning damage.

Necrotic Grasp. The bone monstrosity targets one creature it can see within 60 feet of it. The target must succeed on a DC 23 Constitution saving throw or take 44 (8d10) necrotic damage and have its hit point maximum reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest.

Bone Pull. Bloom Spawns 2d4+2 Ghouls by tearing off a bone and conjuring that many ghouls. She loses hp equal to 5 * the number of bones pulled. 

Get Back Here - Instead of getting an attack of opportunity, when a creature leaves your attack range, suck every creature within 20ft towards you in a necrotic wave.

Legendary Resistance (3/Day). If the bone monstrosity fails a saving throw, it can choose to succeed instead.

To do this you must sacrifice 15 Ghouls nearby:

Spectral Assault(Costs 3 Actions). Bloom releases a wave of spectral energy in a 30-foot cone. Each creature in that area must succeed on a DC 23 Dexterity saving throw, taking 10d10 necrotic damage on a failed save, or half as much damage on a successful one. Each creature also makes a wisdom save of DC 19, if they fail they become paralysed for 3 turns doing a save at the start of each turn.  Upon doing so go into a dormant state for 3 rounds unable to do anything.

Mythic Actions (3/Day). The Bone Monstrosity can take 3 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature's turn. The Bone Monstrosity regains expended mythic actions at the start of its turn.

Mythic Syphon. The Bone Monstrosity makes one slam attack. If the attack hits, the target must succeed on a DC 23 Strength saving throw or be knocked prone and he heals for the same amount.

Mythic Bone Burst (Costs 2 Actions). The Bone Monstrosity releases a burst of necrotic energy in a 30-foot radius. Each creature in the area must make a DC 23 Constitution saving throw, taking 7d6 necrotic damage on a failed save, or half as much damage on a successful one. Bloom heals for the same amount