Legendary Resistance (2/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Devour. This dragon can devour mass and energy to power it self. Giving it 8 Charges

Vaults Commander.  The Vault has given the dragon an ai that allows it to command Automatons and form strategy through this is based on the Vaults knowledge of making them

Vault Possession. The Vault can complete take control over the dragon making it far more competent strategy and in commanding other Vault Automatons

Ice Walk. The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally, Difficult Terrain composed of ice or snow doesn’t cost it extra movement.

Multiattack. The dragon makes three Rend, Bite or Tail attacks. 

Tail. Melee Attack Roll: +14 to hit, reach 15ft., one target. hit 14(2d8 +6) bludgeoning damage

Bite. Melee Attack Roll: +10 to hit, reach 10ft., one target. hit 18(2d12 +6) slashing damage, plus 14 (4d4 + 4) fire damage

Rend. Melee Attack Roll: +14, reach 10 ft. Hit: 15 (1d10 + 10) Slashing damage plus 8 (4d4) Fire damage.

Fire Breath (Recharge 5–6). Dexterity Saving Throw: DC 21, each creature in a 60-foot Cone. Failure: 59 (17d6) Fire damage. Success: Half damage. cost 1 charge

Frost Breath (Recharge 5–6). Constition Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: 54 (12d8) Frost damage. Success: Half damage. cost 1 charge

Burst. The dragon quick absorbs/release an immense amount of energy in an 60ft radius around its self and consuming 3 Charges. If it absorbs energy it will deal (4d8 Cold damage) and freezes the enter area making it difficult terrain and deals 4d4 piercing damage if they move through it .  If it release energy it will deal (6d8 Fire damage) and set fire to the enter area and deals 6d4 Fire damage if they move through it. The dragon will also be forced to stay in place for the rest of the round and will be unable to make legendary action for the rest of the round

Reinforcement. The dragon can reinforce its body with energy adding 4d6 fire or cold damage area of effect of 10ft to any melee attack. cost 1 charge

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

Frightful Presence. The dragon cause throughs around(180ft sphere ) it (below CR3)  (Wisdom Saving Thrown: DC 16) On a fail all being have the Frightened for 5 turns. 

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.