Agni, high fire elemental

Fire aura. Any creature that starts its turn within 30 feet of the agni must succeed on a DC 15 Constitution saving throw or will take 3d10 of fire damage. Items that are easily flamable like clothing, paper or wood ignite and will be consumed in two turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.

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 15 (3d10) 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 15 (3d10) fire damage and catches fire; until someone takes an action to douse the fire, the creature takes 15 (3d10) fire damage at the start of each of its turns.

Illumination. The elemental sheds bright light in a 120-foot radius and dim light in an additional 120 feet.

Legendary Resistance (3/Day). If the elemental fails a saving throw, it can choose to succeed instead.

Spellcasting. The agni is an 18th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). The agni has the following spells prepared:

Cantrips (at will): Fire bolt, Produce flame.

1st level (4 slots): Burning hands, Hellish rebuke.

2nd level (3 slots): Flaming sphere, Heat metal, Scorching ray.

3rd level (3 slots): Fireball, Glyph of warding.

4th level (3 slots): Wall of fire.

5th level (3 slots): Flame strike

6th level (1 slot): (Only for level spells up)

7th level (1 slot): Delayed blast fireball, Firestorm

8th level (1 slot): Incendiary cloud.

9th level (1 slot): Meteor swarm.

Multiattack. The elemental makes two claw attacks.

Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 15 (3d6 + 6) 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.

The elemental can take 2 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The elemental regains spent legendary actions at the start of its turn.

Fan the flames. The agni can sacrifice part of its life to obtain a bonus to all rolls during that round. For every 5 HP the agni sacrifices it gains a +1 to all rolls to a maximun of +10 until the next round.

Inferno (Costs 2 Actions). The agni beats its wings. Each creature within 15 feet of the elemental must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The elemental can then fly up to half its flying speed.