Astral Crocodilian

Antimagic Cone. The astral crocodilian's opened eyes create an area of antimagic, as in the antimagic field spell, in a 120-foot cone. At the start of each of its turns, the crocodilian decides which way the cone faces. The cone doesn't function while the crocodilian's eyes are closed or while the crocodilian is blinded.

Demiplanar Donjon. Any creature or object that the astral crocodilian swallows is transported to a demiplane that can be entered by no other means except a wish spell. A creature can leave the demiplane only by using magic that enables planar travel, such as the plane shift spell. The demiplane resembles a stone cave roughly 100 feet in diameter with a ceiling 20 feet high. Like a stomach, it contains the remains of the crocodilian's past meals. The crocodilian can't be harmed from within the demiplane. If the crocodilian dies, the demiplane disappears, and everything inside it appears around the corpse. The demiplane is otherwise indestructible.

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

Magic Jaws. The crocodilian's attacks are magical, and any offensive Reaction taken by a hostile creature is nullified by the crocodilian's golden gaze. If the crocodilian scores a natural 20 on its bite attack, the targeted creature is swallowed by the crocodilian, suffering the effects of its Demiplanar Donjon feature.

Astral Buoyancy. The crocodilian floats around, hovering in a small localized pool of the Astral Sea, thus being immune to the Prone, Grappled and Restrained conditions and having a Hover (swimming) speed of 40 feet.

Multiattack. The astral crocodilian makes two attacks: one with its bite and one with its claws, or two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 31 (4d10 + 9) piercing damage. 

Claws. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 19 (3d6 + 9) slashing damage. 

The astral crocodilian can take 3 legendary actions, choosing from the options below.  The crocodilian regains spent legendary actions at the start of its turn.

Bite.  The crocodilian makes one bite attack.

Psychic Projection (Costs 2 Actions). Each creature within 60 feet of the astral crocodilian must make a DC 19 Wisdom saving throw, taking 17 (3d10) psychic damage and being knocked prone on a failed save, or just half as much damage on a successful one.

Astral Discharge (Costs 3 Actions, only below half maximum hitpoints). Each creature in a 40 feet cone of the crocodilian must make a DC 19 Dexterity saving throw, taking 55 (10d10) psychic damage and being frightened for 1 minute on a failed save, or just half as much damage on a successful one.