Buggernaut

Restricted Respiration. This creature can go without breathing for one hour.

Spiteful Scent. This creature has advantage on survival checks to track a creature that has dealt damage to it. 

Multi-Attack. This creature makes two attacks, one with its claws and one with its bite.

Action Melee Attack. Claw Attack: +9 to hit, reach 10 ft., 1 target. Hit:  19 (2d12 + 6) [slashing] damage. 

Action Melee Attack.  Bite Attack: +9 to hit, reach 5 ft., 1 target. Hit:  13 (1d12 + 6) [piercing] damage 7 (1d12) [acid] damage. The target is grappled escape DC str 17. It cannot use this attack on another target while it holds the current target grappled in its mandibles and has advantage on attacks it has on a target it has grappled in its jaws.

Chew. While this creature has a creature in its mandibles, it can use a bonus action to chew, automatically inflicting 13 (2d12) piercing damage and 13 (2d12) acid damage. if you do this to a structure or object you can chew through 1 ft of nonmagical material in an environment like metal and rock or anything less durable.

Hunker Down. With a reaction this creature can increase it's AC by 4 and gain resistance to all damage. In this sate, the only actions it can take are its one bite and chew. It takes a full action to exit this state.