Ankheg Terror

Psychic susceptibility. When the Ankheg takes psychic damage it is blinded and can't use its tremorsense ability till the end of its next turn.

Vishous slasher. When the Ankheg deals damage with two attacks or more to a creature, that creature takes 14 (4d6) slashing damage 

Multiattack. The Ankheg uses its Terror trill if available, it makes 1 bite or swallow attack and 2 Claw attacks. 

Terror trill (recharge 5-6). The Ankheg screeches all creatures within 30 feet must make a DC 20 Wisdom saving throw, or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours. Each Ankheg within 60 feet gains a +2 to its attack and damage rolls.

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 31 (4d10 + 7) slashing damage and 14 (4d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 20). Until this grapple ends, the Ankheg can bite only the grappled creature and has advantage on attack rolls to do so.

Swallow. The Ankheg Swallows a creature it has grappled with its bite attack. If the target is a Large or smaller creature, it must succeed on a DC 20 Strength saving throw or be swallowed by the Ankheg. A swallowed creature is blinded  and restrained, has total cover against attacks and other effects outside the Ankheg, 14 (4d6) acid damage and 10 (3d6) acid damage at the start of each of the Ankheg's turns.
If the Ankheg takes 30 damage or more on a single turn from a creature inside it, the Ankheg must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Ankheg. If the Ankheg dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., 1 target. Hit: 35 (4d12 + 7) piercing damage. 

Acid spray (recharge 5-6). The Ankheg spits acid in 3 lines that are 60 feet long and 5 feet wide, provided that it has no creature grappled. Each creature in that line must make a DC 20 Dexterity saving throw, taking 42 (12d6) acid damage on a failed save, or half as much damage on a successful one.

Call the brood. The Ankheg terror calls 1d4 forth Ankhegs to the battle appearing within 30 feet of the terror. 

The Ankheg 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 Ankheg regains spent legendary actions at the start of its turn.

Claw(Costs 1 Action). The Ankheg makes 1 claw attack

Dive (Costs 1 Action). The Ankheg burrows underground and moves its burrow speed, this movement does not provoke opportunity attacks.

Burst forth (Costs 1 Action). The Ankheg Bursts out from underground each creature within 10 feet of the Ankheg must make a DC 20 Dexterity saving throw, on a Success the creature can move to an unoccupied space more than 15 feet away from the Ankheg. On a failure, the creature is moved to an Unoccupied space more than 15 feet away from the Ankheg, knocked prone and takes 14 (4d6) bludgeoning damage. Creatures standing above the Ankheg's space have disadvantage on the saving throw.

Thrash (Costs 2 Actions). The Ankheg shakes around shaking the ground, each creature standing on the ground within 60 feet of the Ankheg must make a DC 20 Constitution saving throw or take 14 (4d6) bludgeoning damage. Creatures riding the Ankheg must make a DC 25 Strength saving throw or be thrown off the Ankheg.