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.

Undead Nature. The dragon does not require air, food, drink, or sleep

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d10 + 2) piercing damage.

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Necrotic Breath. The dragon exhales necrotic gas in a 15-foot cone. Each creature in that area must make a DC 12 Constitution saving throw, taking 22 (5d8) necrotic damage on a failed save, or half as much damage on a successful one.

Slowing Breath. The dragon exhales gas in a 15-foot cone. Each creature in that area must succeed on a DC 11 Constitution saving throw. On a failed save, the creature can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the creature can use either an action or a bonus action on its turn, but not both. These effects last for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself with a successful save.