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

Spellcasting. Yttrius is a 19th-level spellcaster. Its spellcasting ability is [Charisma] (spell save DC 22, +12 to hit with spell attacks). Yttrius has following [class name] spells prepared:
Cantrips (at will): Eldritch Blast, True Strike, Rage Ball
1st level (6 slots): Witch Bolt, Cause Fear
2nd level (5 slots): Crown of Madness, Suggestion
3rd level (5 slots): Counter Spell, Summon lesser demons
4th level (5 slots): Summon Greater Demon, Dimension Door
5th level (4 slots): Modify Memory, Infernal Calling
6th level (3 slots): Summon Fiend, Circle of Death
7th level (3 slots): Power Word: Pain, Plane Shift
8th level (2 slots): Maddening Darkness, Feeble mind
9th level (1 slots):Ture Polymorph, Imprisonment

Magic Resistance. The solar has advantage on saving throws against spells and other magical effects.

Frightful Presence. Each creature of Yttrius' choice that is within 120 feet of Him and aware of it must succeed on a DC 25 Wisdom saving throw or become frightened for 1 minute. 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 Yttrius' Frightful Presence for the next 24 hours.

Demon Scythe. Melee Weapon Attack(Magical): +14 to hit, reach 10 ft.,  Hit: 32 (4d12 + 14) [Slashing] damage 7(2d6) [Necrotic] damage. If the target is a celestial, paladin, or cleric this deals and extra 7(2d6) [Necrotic], In addition when this weapon makes a critical strike is deals 68 (10d12 + 14) [Slashing] + 13 (4d6) [Necrotic]

Rage ball. Ranged Spell Attack: +13 to hit, range 60/120 ft., single target. Hit: 27 (4d10 + 13) [Necrotic] damage.

Second wind (2/day): Heal 73(12d12 + 9)

Parry. As a reaction Yttrius adds +2 to his AC
Retaliation Strike. When hit with a Melee attack Yttrius can do 1 Demon Scythe attack in retaliation. 

Yttrius can take 4 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. Yttrius regains spent legendary actions at the start of its turn.

Deathbang (Costs 3 Actions). Each creature within 40ft. of Yttrius must succeed on a DC 25 Dexterity saving throw. On a failed save, Each creature suffers 12d6 Necrotic damage and be paralyzed until the end of Yttrius' next turn. On a successful save the creature takes half damage and is not paralyzed (2/day)

All-out assault (Costs 3 Actions). Yttrius can move up to his speed wtihout provoking oppurtunity attacks and make 3 Demon Scythe attacks

Attack (Costs 1 Action). Yttrius makes a Demon Scythe attack.

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.