Alduin, Devourer of Worlds

Force without effort. Alduin has advantage on all strength based rolls, is immune to effects that would knock him prone and on every critical strike, he knocks his target prone.

Child of Akkatosh. As a divine being, Alduin is immune to exhaustion and cannot be charmed, petrified or knocked unconscious. Finally, Alduin can't be slain in the mortal realm, he will eventually regenerate. He can, however, be permanently slain in a realm of oblivion, such as the shivering isles or sovngarde. (Forgotten Realms equivalent is any realm other than the material realm.)

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

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Dragon Shouts (Recharge 1-4). Alduin uses one of the following dragon shouts. After which, roll a 1d4 to see when he can use one again.

Fire Breath (Yol Toor Shul). Alduin exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 88 (16d10) fire damage on a failed save, or half as much damage on a successful one.

Frost Breath (Fo Krah Diin). Alduin exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 88 (16d10) cold damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Qo Strun Bah). Alduin exhales lightning in a 90-­-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Unrelenting Force (Fus Ro Dah). Alduin shouts in a 90-foot cone. Each creature in that area must succeed on a DC 24 Strength saving throw or be thrown 20 feet away from the dragon and be knocked prone on a failed saved. The creature also takes 42 (8d10) force damage on a failed save, or half as much on a successful one

Aura Whisper (Laas Yah Nir). For one minute, Alduin detects the presence of living souls within 30ft of him. During this duration, Alduin will see invisible living creatures and see a faint red outline of living creatures through walls. Stealth checks against Alduin are also made at disadvantage during this time.

Dismay (Faas Ru Maar). Every creature within 120 feet of Alduin must succeed on a DC 24 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 succeeds against Dismay, it has advantage on saving throws against Dismay for the next 24 hours.

Drain Vitality (Gaan Lah Haas). Alduin shouts in a 60ft cone, all creatures within that cone must succeed on a DC 24 Constitution saving throw or be effected by this shout. If the creature fails, for one minute, at the start of each of its turns it must make a DC 20 Constitution saving throw or take 22(4d10) necrotic damage. If the creature succeeds, on this save, it takes half as much damage and is no longer affected by this shout. Alduin heals for all damage dealt by this shout.

Elemental Fury (Su Grah Dun). For one minute, Alduin can make three attack actions each turn.

Marked for Death (Krii Lun Aus). Alduin selects a single creature, for one day Alduin will have advantage on any attack roll against them, and the creature will have disadvantage on any saving throws against Alduin.

Slow Time (Tiid Klo Ui). Alduin slows time, for the next 2 rounds of combat, Alduin gains a second initiative roll, and will get a two turns in a round. 

Throw Voice (Zul Mey Gut). For the next minute, Alduin can choose to launch his shouts from any point within 60ft of himself, he is additionally able to freely change the direction of the shouts.

Whirlwind Sprint (Wuld Nah Kest). For the next minute, Alduin gains double walk and fly speed.

Alduin 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. Alduin regains spent legendary actions at the start of his turn.

Regular Attack. The dragon chooses to make either a tail attack, claw attack or bite attack.

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 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Retreat to Sovngarde.

Upon being defeated in the mortal realm, Alduin will flee to the afterlife spirit-world of Sovngarde to devour souls so that he may regain strength.