Ankheg Matt Colville

Eyeballs all over. Immune to flanking.

Multiattack. The ankheg makes three attacks: two with its claws and one with its bite. 

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage plus 3 (1d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, the ankheg can bite only the grappled creature and has advantage on attack rolls to do so.

Claw. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 10 (1d4 + 3) slashing damage. If the target is a Large or smaller creature, and is hit by both claw attacks, it must make a DC 13 Dexterity saving throw, is knocked back 5 ft. and prone on a failed save, or just knocked back on a successful one.

Acid Spray (Recharge 6). The ankheg spits acid in a line that is 30 feet long and 5 feet wide, provided that it has no creature grappled. Each creature in that line must make a DC 13 Dexterity saving throw, taking 10 (3d6) acid damage on a failed save, or half as much damage on a successful one.

Bonus Action

Claw vs Prone. The ankheg can make a claw attack against a target that is prone.

(Optional) The ankheg can take two reactions every turn.

Acid Glob. The ankheg can throw an acid glob at a target that provokes an opportunity attack. Acid Glob: Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: The target must make a DC 13 Dexterity saving throw or it becomes restrained for one round.

Acid for Blood. The first time the ankheg is hit, the ankheg can force its acidic blood out of the wound. Every creature adjacent to the ankhegs wound must make a DC 13 Dexterity saving throw,  taking 6 (2d6) acid damage must make a DC 13 Dexterity saving throw, taking 10 (3d6) acid damage on a failed save, or half as much damage on a successful one.

Acid Death Throws. On every Death Savingthrow the ankheg sprays acid around it. Every creature in a 10 ft. radius of the ankheg takes 6 (2d6) acid damage.

The ankheg can take 1 villain action per round and only at the end of another creature's turn. 

Burrow (Round 1). The ankheg moves up to its burrowing speed without provoking opportunity attacks. If the ankheg has grappled a creature in its mandibles, it is pulled down with it.

Pull Underground (Round 2). The ankheg can burrow under a creature and pull them underground.

Acid Expulsion (Round 3). The ankheg clenches its muscles, bursting small acid filled tubes all over its body. Each creature in a 10 ft. radius must make a DC 13 Dexterity saving throw, taking 10 (3d6) acid damage on a failed save, or half as much damage on a successful one.