Blackveil Ooze

Amorphous. The blackveil ooze can move through a space as narrow as 1 inch wide without squeezing.

Spider Climb. The blackveil ooze can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Thunder Susceptibility. If the blackveil ooze takes thunder damage, it suffers four effects until the end of its next turn: its speed is halved, its blackveil aura range is halved, its blindsight range is halved, and it has disadvantage on saving throws.

Blackveil Aura. All creatures that begin or end their turn within 60 feet of the blackveil ooze have to make a DC 22 Constitution saving throw or take 9 (3d6) poison damage and 9 (3d6) necrotic damage and be poisoned for one minute on a failed save, or half as much damage and not be poisoned on a successful one. The target can repeat the saving throw at the end of its turn, ending the effect on a success. The Miasma generated around the blackveil ooze obscures vision, and all attacks made from a range greater than 10 feet have disadvantage. If a creature dies in the blackveil ooze's blackveil aura they are brought back to life at the start of the next round as a blackveil oozespawn. blackveil oozespawn have the same stat block they did before death, but they change from their previous monster type to both an ooze and undead. The blackveil ooze is capable of telepathically communicating with all oozespawn out to a range of 120 feet. Furthermore, all blackveil oozespawn acquire the same damage, immunities and resistances, condition immunities, and thunder susceptibility trait as the blackveil ooze.

Magic Resistance. The blackveil ooze has advantage on saving throws against spells and other magical effects.

Necrotic Proficiency. The blackveil ooze can concentrate on up to two spells at once as long as one of the spells is a necromancy spell.

Innate Spellcasting. The blackveil ooze’s innate spellcasting ability is Wisdom (spell save DC 23, spell attack bonus +15). It can innately cast the following spells, requiring no components:

at will: bane (5th level), blindness/deafness (5th level), mirror image

3/day each: confusion (5th level), hypnotic pattern, vampiric touch (5th level)

1/day each: mislead, shadow of moil, synaptic static

Multiattack. The blackveil ooze can make three pseudopod and one miasma flare attacks.

Pseudopod. Melee Weapon Attack: +11 to hit, reach 30 ft., one target. Hit: 23 (6d6 + 5) bludgeoning damage plus 20 (5d8) necrotic damage, and the target is grappled if they are large or smaller (escape DC 19). While grappled in this way, the target is restrained as well and the blackveil ooze can spend one of its pseudopod attacks to drag all grappled targets 10 feet closer.

Miasma Flare. Ranged Spell Attack: DC 22 Con save, range 60 ft., three targets. Hit: 12 (3d6 + 5) poison damage plus 12 (3d8) necrotic, and the target is poisoned for one minute. The target can repeat the saving throw at the end of its turn, ending the effect on a success.

Miasmic Irritation (recharge 5-6). A number of creatures equal to the blackveil ooze's wisdom modifier within the ooze's blackveil aura must make a DC 23 Wisdom saving throw or take 9 (3d6) necrotic damage and 9 (3d6) psychic damage and have disadvantage on saving throws while in the blackveil aura for one minute on a failed save. A successful save results in half as much damage and no disadvantage on saving throws. The target can repeat the saving throw at the end of its turn, ending the effect on a success. 

Shapechanger. The blackveil ooze can use its bonus action to polymorph into any Huge or smaller creature it has seen, or back into its true form. Furthermore, it is able to create twice as many copies for each size category below its original (i.e. large = 2, medium = 4, et cetera). If it chooses to create multiple copies of itself, every copy must be within 120 feet of another copy, and they can telepathically communicate. It can create nonmagical gear to help with the validity of the illusion that meld back into its body when it reverts into its true form. It reverts back to its true form upon its death.

Command Oozespawn. The blackveil ooze telepathically commands all oozespawn within 120 feet of it. The commands it can issue are the same as the commands a spellcaster could issue to summons from a spell such as conjure animals.

Mark Corpse. When a creature is killed outside of the blackveil aura's range by a blackveil oozespawn, the blackveil ooze can use its reaction to extend some of its miasma to the corpse, marking it to be turned into a blackveil oozespawn at the start of the next round.

Mark Victim. When the blackveil ooze takes damage, it can use its reaction to mark the target that attacked it. The target is marked for one minute, and while marked all oozespawns will use their turns to move towards and attack the target unless commanded otherwise, and the target receives half as much health from healing spells. If the target is killed while suffering from mark victim, they are immediately resurrected as a blackveil oozespawn. The target can make a DC 22 Constitution saving throw at the end of its turn to attempt to end the effect on a success. 

The blackveil ooze can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The blackveil ooze regains spent legendary actions at the start of its turn.

Attack. The blackveil ooze makes one pseudopod or miasma flare attack.

Disengage. The blackveil ooze takes the disengage action and moves up to half of one of its movement speeds.

Spellcast (Costs 2 Actions). The blackveil ooze casts a spell.