Bombling

Agressive. As a bonus action, the elemental can move up to its speed toward a hostile creature that it can see.

Body of Fire. This elemental has a spongey physical form containing a fiery core, any damage it takes, except cold, causes all creatures winthin 5 feet of it to make a DC 15 Dexterity saving throw, taking 4(1d8) fire damage on a failed save. Additionally, Body of Fire causes the elemental to sheds bright light in a 20-foot radius and dim light in an additional 20 feet while active, and 20 feet of dim light while inactive.

Doused Flames. If the elemental takes cold damage, it must make a Constitution saving throw DC equal to the cold damage taken or Body of Fire ceases to function until the end of its next turn. Additionally, for every 5 feet the elemental moves in water, or for every gallon of water splashed on it, it takes 1 cold damage.

Death Burst. If the elemental reaches 0 hit points it's fiery core explodes. Each creature within 20 feet of the elemental must make a DC 15 Dexterity saving throw, taking 21(6d6) fire damage on a failed save, or half as much on s successful save. This ability cannot trigger if Body of Fire is not active.

Ember Reborn. 24 hours after the elemental uses its Death Burst ability it reconstitutes at full hit points.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 14 (3d8 + 3) piercing damage plus 4(1d8) fire damage.

Fire Breath (recharge 6). The elemental expells a 20 feet cone of fire. Each creature within the area must make a DC 15 Dexterity saving throw, taking 21(6d6) fire damage on a failed save, or half as much on a successful save.

Swell. Whenever the elemental takes damage it can use its reaction to gain one charge of Swell. All charges of Swell go away once the elemental is no longer engaged in combat. If it's Death Burst ability activates, increased the damage dealt by 2d6 per charge of Swell. If the elemental gets 3 or more charges of Swell, it's Death Burst immediately activates killing the elemental.