Legendary Resistance (3/long rest). Aaren can chose to succeed a failed saving throw three times per long rest. 

Spell Redirection. When Aaren uses a legendary resistance to save from a spell effect, he suffers no effects even if he would usually take half. Instead, he redirects the spell towards a creature within 60 feet. The target of Aaren's redirection must make a saving throw against the original caster's spell save DC or suffer the effects.

Platinum Armor (1/day). Aaren can use a bonus action to activate the armor, which grants him resistance to acid, cold, fire, lightning, and poison damage for 1 minute. Additionally, he gains the Evasion ability.

Warcaster. Aaren has the warcaster feat which gives him advantage on concentration checks, allows him to cast somatic spells with a weapon, and he can cast a spell as an attack of opportunity.

Mobile. Aaren's speed increases to 40 ft. and he does not provoke opportunity attacks. Additionally, he can move as part of his legendary actions, but he can only move up to his walking speed per round.

Spellcasting. Aaren is a 12-level spellcaster. His spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks). Aaren has the following spells prepared:

Cantrips (at will): control flames, fire bolt, mage hand
1st level (4 slots): detect magic, catapult, silvery barbs
2nd level (3 slots): enlarge/reduce, misty step
3rd level (3 slots): counterspell, dispel magic, slow
4th level (3 slots): wall of fire
5th level (2 slots): bigby's hand, mislead
6th level (1 slots): chain lightning

Multiattack. Aaren d'Cannith makes two Arm Blade attacks. Additionally, if Aaren rolls a 1 or 2 with his arm blade damage, he can reroll and must use the new roll.

Arm Blade. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) slashing damage. 

Spellfire Armor. When Aaren is hit with an attack from a magical weapon, his armor absorbs some of the magical energy. For every strike, one fragment of spellfire energy is collected. The armor can store up to 10 fragments before they must be released. As an action or legendary action, Aaren releases the energy in a 10-ft radius sphere centered on him. All creatures in the radius must make CON saving throws or take force damage equal to 1d6 times the number of fragments released. On a successful save, the creature takes half damage. 

Aether Infusion. When Aaren makes an attack with his arm blade, he can use a bonus action to expend a spell slot and invoke an aether infusion that lasts for one Multiattack action. He can only use one infusion per round.

• Blazing Strike. Expend a 1st level spell slot to add 3 (1d6) fire damage.
• Arcane Force. Expend a 2nd level spell slot to add 7 (2d6) force damage. On a hit, the creature must succeed a Strength save or be pushed 15 ft away from Aaren.
• Blinding Blast. Expend a 3rd level spell slot to add 14 (3d8) radiant damage. On a hit, the creature must succeed a Constitution save or be blind until the end of Aaren's next turn.

Aaren has three legendary actions per round, which he regains at the start of his turn.

Summon Warforged (2 Actions). Aaren summons 2 (1d4) warforged that obey his command and take their turn after his own.

Blade Arm Attack (1 Action). Aaren makes an attack with his Blade Arm.

Spellfire Surge (1 Action). Aaren releases the spellfire armor's stored energy.