Birth of Horror

Horrid Gnashing. The Birth of Horror's mouth gnashes incoherently while it can see any enemies. Each creature that starts its turn within 20 feet of the nullifier and can hear it must make a DC 14 Wisdom saving throw. Unless the save succeeds, the creature rolls a d8 to determine what it does during the current turn:

1–4: The creature is stunned until the end of the turn.

5–6: The creature is frightened until the end of the turn and uses its movement to get as far as possible from the nullifier.

7–8: The creature doesn’t move, and it uses its action to make one melee attack against a random creature (other than itself) if one is within reach. It otherwise does nothing.

Innate Spellcasting. The Birth of Horror's innate spellcasting ability is Charisma (spell save DC 16). It can innately cast the following spells, requiring no material components:

At will: counterspell (see “Reactions” below), detect magic, dispel magic, see invisibility

1/day: antimagic field

Magic Resistance. The Birth of Horror has advantage on saving throws against spells and other magical effects.

Multiattack. The Birth of Horror makes three attacks: one with its bite and two with its claws.

Bites. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 17 (2d12 + 4) piercing damage plus 11 (2d10) force damage, and the target is grappled (escape DC 16) if it’s a creature. Until this grapple ends, the target is restrained and the Birth of Horror can't bite another target.

Claws. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 11 (2d10) force damage.

Swallow: The Birth of Horror makes one bite attack against a Medium or smaller target it is Grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is Blinded and restrained, it has total cover against attacks and other effects outside the Horror of Birth, and it takes 14 (2d12) acid damage at the start of each of the Horror of Birth's turns. The Horror of Birth can have only one target swallowed at a time.
If the Horror of Birth dies, a swallowed creature is no longer restrained by it and can Escape from the corpse using 5 feet of movement, exiting prone.

Horrifying Visage. Each non-undead creature within 60 feet of the Horror of Birth that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, with disadvantage if the Horror of Birth is within line of sight, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the Horror's of Birth's Horrifying Visage for the next 24 hours.

Counterspell. The Birth of Horror attempts to interrupt a creature that it can see within 60 feet in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, the Horror of Birth makes a Charisma check with a DC equal to 10 + the spell’s level. On a success, the creature’s spell fails and has no effect.

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

The legendary monster can take 3 legendary actions. Only one legendary action option can be used at a time and only at the end of another creature's turn. The legendary monster regains spent legendary actions at the start of its turn.

Claws Attack. The Horror of Birth makes one Claws attack.

Bites Attack. The Horror of Birth makes a Bites attack.