When Ankheg Rider Dorrinn falls below half health his Scorched Ankheg dies and Ankheg Rider Dorrinn becomes a medium creature with a base move speed of 40 ft. And an AC of 21. melee Weapon attacks made by Ankheg Rider Dorrinn now deal an additional 2d6 slashing damage.
Multi attack: Ankheg Rider Dorrinn may make two attacks with his Scimitar or Shortbow and use either Bite or Blister Bomb if he is a large creature. Scimitar. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6+4) Slashing damage. Savage: If this weapon would roll a natural 15 or higher, it deals an additional 1d4 weapon damage. Short Bow. Ranged Weapon Attack: +8 to hit, range 30 ft./120 ft. one target. Hit: 7 (1d6+4) Slashing damage. Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 15 (3d8+3) slashing damage plus 2 (1d4) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, Ankheg Rider Dorrinn can bite only the grappled creature and has advantage on attack rolls to do so. Caustic Bomb (Recharge 6). The Scorched Ankheg being rode spits a ball of mucus at an area up to 60 ft. away. Upon landing, the mucus ball explodes in a sphere with a 10 ft. radius. Creatures within this sphere must make a DC 16 Dexterity saving throw, taking 15 (6d6) damage on a fail and half as much on a successful save. Sprinting Whirlwind. If Ankheg Rider Dorrinn is considered a medium sized creature he moves up to 30 ft. in a line, making a Scimitar attack against any creature within 10 ft. of this line