Bound Undead Dragon

Amphibious. The undead dragon doesn't need to breathe eat or sleep

Parasitic. The undead dragon is bound to the necromancer that raised it if the necromancer is killed the Dragon will also die, in addition the necromancer that is bound to the dragon can choose  to drain unto 4d10+6 Health points from the dragon on the dragons turn.

Undead Fortitude. If damage reduces the Dragon to 0 hit points, it must make a Constitution saving throw with a DC of 5+the damage taken, unless the damage is radiant or from a critical hit. On a success, the Dragon drops to 1 hit point instead.

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

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 19 (2d12 + 6) piercing damage 

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Necrotic Breath (Recharge 5–6). The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 44 (10d8) necrotic damage on a failed save, or half as much damage on a successful one.

The dragon can take 3 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 dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 17 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.