Unusual Nature. The undead dragon tail doesn't require air, food, drink, or sleep.

Turn Resistance. The undead dragon tail has advantage on saving throws made against effects that turn undead.

 Undead Fortitude. When the undead dragon tail drops to 0 HP, it instead drops to 1 hp, unless it the damage was caused by a critical hit or was radiant damage.

Multiattack. The undead dragon tail makes one Claw, Tail, and Wing Attack.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft. one target. Hit: 8 (1d6 + 5) slashing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained. The dragon has two claws, each of which can grapple one target.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Wing Attack. The undead dragon tail beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The undead dragon tail can then fly up to half its flying speed.

The Undead Dragon Tail may take one Legendary Action at the end of another creature's turn. During that Legendary Action, it may make one Claw or Tail attack against a creature in its range. Alternatively, it may:

Tail Sweep. The Undead Dragon Tail sweeps its tail, affecting all creatures within a 15ft-cone. All affected creatures must make a DC 19 Dexterity saving throw or be knocked prone and take 4d4 bludgeoning damage, taking no damage on a successful save.