Adult Red Dragon (Undead)

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

Legendary Resilience (1/Week). When the dragon is reduced to zero HP, it instead falls to the ground unconscious with 1 HP for one round. After the 6 seconds, the dragon awakens with 25 HP and resistance to all damage for one round.

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

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 29 (4d10 + 8) piercing damage plus 13 (4d6) fire damage.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 21 (4d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 23 (4d8 + 8) 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 21 Wisdom saving throw or become frightened for 1 minute. A creature within 30  feet of the Dragon has Disadvantage on this Saving Throw. 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.

Necro-Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire/necrotic damage on a failed save, or half as much damage on a successful one.

Tail Slam. The Dragon can make one Tail Attack as a Reaction. A creature hit by this attack of opportunity must pass a DC 21 Dexterity Saving Throw or be knocked Prone. 

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 30 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 21 (4d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.