Argathaz the Undying

While Argathaz is conscious, and within 100ft, he can control undead he is aware of without spending an action or bonus action.

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 4 (1d8) acid damage.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Necro Breath (Recharge 5–6). The dragon exhales necrotic ichor in a 15­-foot cone. Each creature in that cone must make a DC 14 Constitution saving throw, taking 49 (11d8) necrotic damage on a failed save, or half as much damage on a successful one.

Spawn Servant

As a bonus action on each of his turns, Argathaz can create an undead creature at a point he can see within 30 feet of himself. This undead creature must be no greater than CR 1/2, and must roll initiative once created. If this creature is within Argathaz's Necro Breath attack, the instead take that damage as healing. 

Aura of Death  

 When Argathaz takes damage from a magical attack, he can change that damage type to necrotic.

Argathaz 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. Ancient Red Dragon regains spent legendary actions at the start of their turn.

Urgent Army (Costs 2 Actions)

Argathaz can use his Spawn Servant ability. 

Fly

 Argathaz can fly directly up 30 feet without provoking opportunity attacks.