Alduin

Amphibious. Alduin can breathe air and water.

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

Multiattack. Alduin can use its Frightful Presence. He then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) acid damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft.,  Causes shockwave to spread in a 15ft. Hit: 17 (2d8 + 8) bludgeoning damage. Shockwave:  7 (1d10+2)

Frightful Presence. Each creature of Alduin's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 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 Alduin's Frightful Presence for the next 24 hours.

Fire Breath (Recharge 5–6). The dragon exhales Fire in a 90-­-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) Fire damage on a failed save, or half as much damage on a successful one.

Frost Breath (Recharge 5–6). Alduin exhales Frost in a 90-­-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) Frost  damage on a failed save, or half as much damage on a successful one. If the shout hit any creature its speed is reduced by 2(1d4).

Unrelenting force (Recharge 6-7): Alduin releases a powerful breath that reaches 90 feet forward and 10 feet wide. Each creature hit must make a DC 25 Dexterity check or be knocked back 10 feet and be knocked  prone. On a success the creature goes back 5  feet.

Meteor Breath (1/battle):  Alduin Releashes a powerful roar causing meteors to rain from the sky. Each player and NPC other than Alduin and anyone he choses to be immune to it has a 15% chance to be hit by a meteor causing 5 (1d8+2) Bludegoning damage.

Armor Strengthen: If alduin is attacked at a single point he can strengthen that point of his Dragon skin and reduce damage by 6 (2d6).

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). Alduin beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.