Ancient Red Dragon (Undead)

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

Death Throes(Upon Defeat). When the dragon is reduced to zero HP, it instead falls to 1 HP and lets out a violent surge of necrotic flames. All creatures in a 30 ft radius of the Dragon, must pass a DC 25 Dex Saving Throw or take 40 (13d6) fire/necrotic damage on a failed save, or half as much damage on a successful one. This damage cannot be reduced. 

Multiattack. The dragon can use its Frightful Presence. It then makes four attacks: one bite and any other 3 attacks. 

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 31 (4d10 + 10) piercing damage plus 24 (8d6) fire damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 23 (4d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 27 (4d8 + 10) 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 25 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 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) 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 25 Dexterity Saving Throw or be knocked Prone.

Give Chase. When a creature in melee with the Dragon moves more than 20 feet away, it can move up to half of its Flying Speed toward that creature without provoking Opportunity Attacks as a Reaction .

Breath Shield. The Dragon cannot use more than 1 Legendary Action in the same round it uses its Breath Shield. When the Dragon would take damage as a result of failing a Saving Throw against a spell, it can immediately regain and expend a use of its Breath Weapon, reducing the incoming damage by the amount rolled. 

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 two tail attacks.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 23 (4d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.