Avatar of Flame

True Flame: Before each sword attack the target needs to make a Wisdom saving throw DC equal to your spellcasting modifier. On a faild save target looses their fire resistance untill attacker next turn, or his fire immunity is reduced to resistance. 

Fire Form. The elemental can move through a space as narrow as 1 inch wide without squeezing. A creature that touches the elemental or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage. In addition, the elemental can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 5 (1d10) fire damage and catches fire; until someone takes an action to douse the fire, the creature takes 5 (1d10) fire damage at the start of each of its turns.

Illumination. The elemental sheds bright light in a 30-foot radius and dim light in an additional 30 feet.

Water Susceptibility. For every 5 feet the elemental moves in water, or for every gallon of water splashed on it, it takes 1 cold damage.

Multiattack. The elemental makes two attacks.

Touch. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) fire damage. If the target is a creature or a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.

Sword of Promise. Melee spell atack +9 to hit, reach 5 ft, one target. Hit: 4d6+4 fire damage.

If the spell was cast using higher level spellslot when shapeshifting, the damage increases by 2d6 for every two slot level above 2nd.