Ancient Thrax

Sunlight Sensitivity. While in sunlight, the ancient thrax has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Multiattack.  The ancient thrax makes three Fist attacks.

Fist.  Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 22 (4d10+5) bludgeoning damage.

Water Drain. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 16 (4d8) necrotic damage and the creature is grappled (escape DC 18). Until the grapple ends, the target takes the same damage at the beginning of its next turn.  The ancient thrax cannot use Water Drain on more than one target.

A creature that escapes the grapple must make a DC 18 Constitution saving throw. On a failure, the creature becomes afflicted with the thrax curse, transforming into a thrax over 2-4 weeks unless the curse is removed. A creature reduced to 0 hit points by Water Drain is killed, leaving only a desiccated husk.

Baleful Glare (Recharge 5-6). The Ancient Thrax targets one creature it can see within 60 feet. The target must succeed on a DC 18 Constitution saving throw or be stunned until the end of its next turn.

Shadow Walk.  As a bonus action, the ancient thrax can step into shadow and teleport up to 30 feet to an unoccupied space in dim light or darkness. While in shadows, the thrax is invisible until it attacks or until its concentration ends (as if concentrating on a spell).

Displacement. When the ancient thrax is targeted by a melee or ranged weapon attack, it can use its reaction to create a psionic displacement effect, causing the attack to have disadvantage on the attack roll, as the thrax becomes blurred and difficult to pin down.

Energy Containment. When the ancient thrax is targeted by a spell or effect that deals fire, cold, lightning, or radiant damage, it can use its reaction to absorb and contain that energy. It instead takes half-damage and stores the remaining energy. On its next successful melee attack the thrax releases that energy, dealing the other half of the absorbed damage to its target in addition to its melee damage. The stored energy dissipates harmlessly if it is not used within one minute.

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

Shadow Step (1 Action). The ancient thrax uses its Shadow Walk ability to teleport up to 30 feet to an unoccupied space in dim light or darkness. This movement does not provoke opportunity attacks.

Crushing Fist (1 Action). The ancient thrax makes a single Fist attack. If the attack hits, the target must succeed on a DC 18 Constitution saving throw or have disadvantage on its next attack roll as it reels from the blow. 

Call of the Black (2 Actions). The ancient thrax targets one creature it can see within 30 feet. The target must succeed on a DC 18 Strength saving throw or be pulled into the Black for 1 round. While in the Black, the creature takes 27 (6d8) psychic damage, is incapacitated, and cannot perceive the outside world. At the end of the thrax's next turn, the creature reappears in an unoccupied space within 5 feet of where it was pulled in.