Reactive Chitin. The Abberankhegs body has been distorted and displaced, it's form far larger, more purple, and bizarrely bulbous. Whenever a melee, ranged or spell attack is made on the Abberankheg, if that attack hits, the total roll of that attack becomes the Abberankhegs armor class until the start of it's next turn, after taking damage from that attack which hit. 

Multiattack. The Abberankheg makes 2 Tentacle Mutation attacks, and one Bite attack. 

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 6) slashing damage plus 5 (2d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 16). Until this grapple ends, the abberankheg can bite only the grappled creature and has advantage on attack rolls to do so.

Tentacle Mutation. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. If the target is a Huge or smaller creature, it is grappled (escape DC 16). 

Acid Spray (Recharge 5-6). The abberankheg spits acid in 2 lines that are 30 feet long and 5 feet wide each. Each creature in that line must make a DC 16 Dexterity saving throw, taking 12 (4d6) acid damage on a failed save, or half as much damage on a successful one.

Extra Mouth. Whatever alien corruption has mutated this creature has given it an extra mouth on the back of it's head. Whenever a creature exits or enters the abberankhegs melee range, and the abberankhegs use of Acid Spray has not recharged, the abberankheg can use it's Acid Spray action. If it chooses to use this reaction, it cannot use it's Acid Spray again until it's next long rest.