Magic Resistance. The deva has Advantage on saving throws against spells and other magical effects.

The Flames of War. The deva releases a 10 ft. aura that forces all creatures inside to take 3d6 fire upon entering for the first time in a round and should they end their turn in it.

Ride the Fire (only available if riding a Nightmare). The deva's steed rides upon the stream of fire emanating from its rider's body. Use the Nightmare's walking speed and fly speed when moving this creature. Consider the Nightmare to be constantly dashing. Furthermore, creatures who have capability of being ridden and are immune to fire can also ride the flames generated by the deva. 

The Second Trumpet. If Abaddon - Angel of War would be reduced to 0 hit points, its current hit point total instead resets to 115 hit points and it regains any expended uses of Legendary Resistance. Additionally, the deva can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 13000 XP (26,000 XP total) for defeating the deva after The Second Trumpet activates.

Multiattack. The deva makes two Spear of War attacks or Flaming Burst.

Spear of War. Melee Weapon Attack: +14 to hit, reach 10 ft., # target. Hit: 11 (2d6 + 8) [Piercing] damage plus 18 (4d8) [Fire] damage.

Doom Helm (Recharge 6). The deva manifests a beam of energy shot from the opening in its helm in a 300ft long 10ft wide line. Plant life and creatures CR or Level 3 and below are destroyed instantly. Creatures that do not die must make a DC 18 Constitution Saving Throw. Failure: The creature takes 49 (14d6) fire and if this would bring their hit points to 0, they are instantly destroyed. Success: The creature takes half damage and is not destroyed.

Sound of the Trumpet (The Second Trumpet only. Once per long rest). All creatures within 150 ft. that can hear the deva must make a DC 18 Constitution Saving Throw. Failure: The creature takes 35 (10d6) Fire, is restrained until the end of its turn, and suffers 3 levels of exhaustion.. Success: The creature takes half damage and suffers 2 levels of exhaustion.

Flaming Burst (The Second Trumpet only). Melee and Ranged Weapon Attack: +14 to hit, reach 10 ft., # target. Hit: 32 (4d12 + 6) [Fire] damage.

Spellcasting. The deva casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 18):

At Will: Detect Evil and Good, Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell), Fireball

1/Day Each: Commune, Raise Dead

Divine Aid (2/Day). The deva casts Cure Wounds, Lesser Restoration, or Remote Curse, using the same spellcasting ability as Spellcasting.

Hellish Rebuke. The deva casts Hellish Rebuke at 5th level. Saving throw is DC 18.

Legendary Action Uses: 3. Immediately after another creature’s turn, the deva can expend a use to take one of the following actions. The deva regains all expended uses at the start of each of its turns.

Banner of War. The deva summons two spirits of war (use fire elemental stat blocks with hit points reduced to 1). If the deva is in mounted combat, being chased, the spirits fly towards the closest creature behind and take their turns. The spirits of war disappear at the start of the deva's next turn and can continue to take actions every legendary action.

Roar! The Flames of Hate! The deva releases a swarm of fireballs in a 60ft cone behind it. Creatures inside the cone must make a DC 18 Dexterity Saving throw. Failure: The creature takes 14 (4d6) Fire and is on fire taking 2d6 fire at the start of each of their turns until put out. Success: The creature takes half damage and is not ignited.

Burn Up and Die! The Spears of War (2 Actions).  The deva creates a set of flaming spears in the air acting as footholds for its Nightmare to ride upon. The deva moves half of its movement speed raising its spear into the sky before launching the flaming spears at up to 3 creatures within 120 ft. Make a Spear of War attack against each. On a hit, they take critical damage and are knocked prone, if mounted the mount is also knocked prone.

When The Second Trumpet is activated, the deva gains the following abilities as replacement for its Legendary Actions options.

Wrath. The deva casts delayed blast fireball. When casting this spell the deva does not need a physical point. The fireball will detonate wherever the deva chooses.

The Call of War. The deva chooses 2 creatures within sight, each creature must make a DC 18 Wisdom Saving Throw. Failure: The creature must attack the other creature affected until the end of its turn. Success: No effect. (this is considered a charm).

Overheat (Costs 2 Actions). The deva emanates a 20 ft. radius of fire. All creatures inside must make a DC 18 Constitution Saving Throw. Failure: The creature gains a point of exhaustion and becomes vulnerable to fire damage until the end of the deva's next turn. Success: The creature is still vulnerable but does not gain exhaustion.