Burnshell

Fire Up. The living flame that inhabits a Burnshell's armour burns brighter in the absence of disruption. At the end of each of its turns, the Burnshell ignites! It gains an additional attack per round with either its longsword or its longbow, an additional 10 ft. to its walking speed, and an extra +2 to all damage rolls. If the Burnshell takes damage while ignited or uses its Blasting Strike, it immediately loses all benefits of this trait until it ignites again. 

Mark of Baator. The Burnshells are prideful mercenaries who obey only those with the authority to command them, and detest would-be usurpers to unearned power. A Burnshell subjected to any charm effect immediately knows that it has been charmed, and can choose at any point during the duration of the charm to self-immolate, immediately reducing itself to 0 HP and reforming somewhere in Avernus. A Burnshell will only resort to this if given an order it considers degrading or disrespectful - otherwise, it will happily wait out its forced inaction until such a time as it can administer a fitting comeuppance to the mageling that dared try to control it.

Multiattack. The Burnshell makes one attack with either its longsword or its longbow, or uses Blasting Strike.

Longsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) Slashing damage, or 8 (1d10 + 3) slashing damage if used with two hands.

Longbow. Ranged Weapon Attack: +6 to hit, range 150/600 ft., one target. Hit: 7 (1d8 + 3) Piercing damage.

Blasting Strike (1/day). Once per day while under the ignited effects of Fire Up, the Burnshell can focus its wildfire spirit into one attack. The Burnshell makes one attack with its longsword against a target within reach. If the attack is successful, the target suffers an additional 12 (3d8) Fire damage and must succeed a DC 15 Strength saving throw or be knocked 10 ft. away from the Burnshell. Additionally, any flammable items worn or carried by the target catch fire. If the attack does not hit, this attack can be reused on the Burnshell's next turn and it does not lose the benefits of Fire Up.