Vaal'ral, The Dead Lightwarden

Amphibious. The dragon can breathe air and water.

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Colossal. The dragon is considered Colossal sized (on grid: 60 ft. by 60 ft. or larger). This means that opportunity attacks made against it and its own opportunity attacks are made with disadvantage. 

Extreme Challenge. Orkuukl'ralusk's actual CR is 54 (29,811,000 XP,  Proficiency Bonus: +14).

Immortal Vitality. Vaal'ral doesn't suffer any effects from aging, doesn't age, and doesn't require food, drink, air, or sleep. Also whenever Vaal'ral takes a short rest, he regains an additional 2d8 hit points.

Grim Reaper. When Vaal'ral casts a Necromancy spell of 1st through 7th-level that targets only one creature, the spell can instead target two creatures within range and within 5 feet of each other. If the spell consumes its material components, he must provide them for each target.

Channel Divinity: Touch of Death (5/Short or Long Rest). When Vaal'ral hits a creature with a melee attack, he can deal an additional 32 necrotic damage to the target.

Inescapable Destruction. Necrotic damage dealt by Vaal'ral's cleric spells and Channel Divinity options ignores resistance to necrotic damage. Additionally, creatures that do not have resistance or immunity to necrotic damage take damage dealt by Vaal'ral's cleric spells and Channel divinity options as if they have vulnerability to necrotic damage.

Blessed Strikes (2/Round). When a creature takes damage from one of Vaal'ral's cantrips or weapon attacks, he can deal an additional 1d8 necrotic damage to that creature.

Sacred Spells. Whenever Vaal'ral expend a spell slot to cast one of your Domain spells of 4th level or lower, Vaal'ral may instead cast that spell at 5th level without expending a higher level spell slot.

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

Bite. Melee Weapon Attack: +29 to hit, reach 30 ft., one target. Hit: 30 (4d6 + 15) piercing damage + 7 (2d6) fire damage + 14 (4d6) necrotic damage.

Claw. Melee Weapon Attack: +29 to hit, reach 25 ft., one target. Hit: 26 (2d10 + 15) slashing damage + 5 (1d10) necrotic damage.

Tail. Melee Weapon Attack: +29 to hit, reach 35 ft., one target. Hit: 29 (2d12 + 15) bludgeoning damage + 1 necrotic 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 29 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 the dragon's Frightful Presence for the next 24 hours.

Golden Gaze (Recharge 6). The dragon's eyes start to glow like the sun and it focuses its gaze on one creature it can see within 120 feet. The creature has to make a DC 30 Constitution saving throw, taking 38 (7d10) radiant damage and being blinded until the end of its next turn on a failed save and taking half as much damage and not being blinded on a successful one. The dragon also must make this save but with advantage and with a DC of 19 or take 5 (1d10) radiant damage.

Horrific Gaze. The dragon's eyes glow like a star covered by a black eclipse and it focuses its gaze on one creature it can see within 120 feet. The creature has to make a DC 29 Constitution saving throw, taking 16 (3d10) radiant damage + 22 (4d10) necrotic damage and being blinded until the end of its next turn on a failed save and taking half as much damage and not being blinded on a successful one.

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

Fire Breath. The dragon exhales fire in a 100-foot cone. Each creature in that area must make a DC 29 Dexterity saving throw, taking 115 (21d10) fire damage on a failed save, or half as much damage on a successful one.

Weakening Breath. The dragon exhales gas in a 100-foot cone. Each creature in that area must succeed on a DC 29 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Innate Spellcasting. The dragon's spellcasting ability is Charisma (spell save DC 27, +18 to hit with spell attacks). The dragon can innately cast the following spells, requiring only verbal components:

At Will: control flames, blade ward, guidance, cure wounds, shield, fireball, zone of truth, suggestion, bless, wall of fire

2/day each: slow, dispel magic, geas, commune, greater invisibility, plane shift, word of recall, globe of invulnerability, teleport, sunburst

5/day: hold monster, immolation, maze, foresight

2/day: antimagic field, true resurrection

1/day: lesser wish, gate guardian

Spellcasting. Orkuukl'ralusk is a 50th-level spellcaster. His spellcasting ability is Wisdom (spell save DC 32, +24 to hit with spell attacks). He has the following cleric spells prepared:

Cantrips (at will): light, spare the dying, thaumaturgy, toll the dead, guidance, mending

1st level (at will): false life, ray of sickness, bane, detect magic, protection from evil and good, create or destroy water, command, bolster undead, anticipate weakness

2nd level (at will): blindness/deafness, ray of enfeeblement, silence, zone of truth, augury, borrowed knowledge, ghostly shackles, hold person

3rd level (7 slots): animate dead, vampiric touch, bestow curse, dispel magic, spirit guardians, speak with dead, feign death, glyph of warding

4th level (6 slots): death ward, blight, banishment, stone shape, divination, freedom of movement, control water, locate creature

5th level (6 slots): antilife shell, cloudkill, greater restoration, hallow, planar binding, insect plague, contagion, raise dead

6th level (6 slots): circle of death, harm, heal, curse of undeath, find the path, true seeing, forbiddance, create undead, divine facade

7th level (5 slots): etherealness, finger of death, regenerate, divine word, symbol, temple of the gods

8th level (5 slots): Abi-Dalzim's horrid wilting, mind blank, earthquake, control weather, antimagic field, desolation

9th level (5 slots): power word kill, heart ripper, gate, astral projection, reanimation

10th level (4 slots): absolute death, doom, antimagic ray, divine's sight

11th level (4 slots): determinism, black death, eternal freedom 

12th level (3 slots): soul harvest, eidolon

13th level (2 slots): phenomenal cosmic power

14th level (1 slot): personal plane

15th level (1 slot): true binding

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 25 feet of the dragon must succeed on a DC 29 Dexterity saving throw or take 21 (2d6 + 15) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.