Ancient Red Dragon of Fury

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes two Rend attacks. It can replace one attack with a use of Spellcasting to cast Scorching Ray (level 3 version).

Rend. Melee Attack Roll: +15, deal 2d6 + 5 slashing damage and 1d6 fire damage. When a player is hit with a rend attack, they must make a Constitution saving throw or be ignited. When ignited, at the start of your turn, take 2d4 fire damage. You must take an action to put yourself out, or someone else can take an action to remove the condition from you.

Fire Roar(Recharge 5–6):  Everyone within a 5ft radius must make a Constitution saving throw or become ignited and take 2d4 fire damage. If you succeed, you take 2d4 fire damage and do not become ignited. The range increases by 5ft for every 15 health points missing.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 23, +15 to hit with spell attacks):

At Will: Command (level 2 version), Detect Magic, Scorching Ray (level 3 version)
1/Day Each: Fireball (level 5 version), Scrying

Once per long rest and if the dragon is bloodied, as a bonus action, the dragon enters a state of ancient fury. In this state, the dragon heals from fire damage and receives half damage from all damage sources except cold, necrotic, and radiant. In this form, all of its attacks also deal an additional 1d4 fire damage, and it heals for half of any fire damage it deals on its turn. 

When hit with a melee attack, the dragon can flap its massive wings as a reaction to push all creatures 15 feet away, and until the end of the turn, that area around the dragon is considered difficult terrain. 

Legendary Actions (3 actions): 

Crown of Fury (2 actions): The target must make a Wisdom saving throw, or a crown of burning flames is put on their head, and the target is charmed by the dragon for 1 minute and takes 2d4 fire damage at the start of each of its turns. If the dragon damages the creature in any way outside of the passive damage from the crown, the charm ends, and the crown fades away into ash.

Dash (1 action): The dragon can move up to half of its move speed without provoking attacks of opportunity.

Flare (1 action): Target one creature and make them roll a dexterity saving throw or become ignited.

Supernova (3 actions): Use Fire Roar at a set range of 30 ft, but instead deals 1d4 for every 20 health missing.