Bahamut, Derealization

Demigod's Vitality: The dragon's ability scores and hp maximum can't be reduced except by means of a wish spell.

Material Bound: The dragon is immune to any spell or affect that would force it to teleport or leave the current plane.

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

Hateful Aura: The dragon adds its Charisma modifier to all of its melee weapon attacks (included in the attacks). 

Conqueror's Aura: If a creature is frightened of the dragon, its speed is reduced to 0 while in the aura, and that creature takes 15 psychic damage if it starts its turn there. The aura extends 60 feet from the dragon in every direction, but not through total cover.

(Dragon Form) Multiattack. The aspect makes one Bite attack, one Claw attack, and one Tail attack.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 33 (2d12 + 20) piercing damage plus 22 (4d10) force damage. The target must succeed on a DC 27 Wisdom saving throw or become frightened of the aspect until the end of the target’s next turn.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 31 (2d10 + 20) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. The aspect can have only one creature grappled this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 33 (2d12 + 20) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Time Breath (Recharge 1–2). 

The dragon exhales a wave of shimmering light in a 300-foot cone. Nonmagical objects and vegetation in that area that aren’t being worn or carried crumble to dust. Each creature in that area must make a DC 24 Constitution saving throw. On a failed save, a creature takes 52 (8d12) force damage and is magically weakened as it is desynchronized from the time stream. While the creature is in this state, attack rolls against it have advantage, it has the poisoned condition, and other creatures have resistance to all damage it deals. On a successful save, the creature takes half as much damage only. A weakened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves.

Gravitic Breath. (Recharge 3-4). 

The dragon exhales localized gravity in a 300-foot cone. When a creature ends a fall in the cube, it takes 1d10 bludgeoning damage for every 10 feet it fell, to a maximum of 20d10. A creature that enters the cone for the first time on a turn or starts its turn there must make a DC 24 Dexterity saving throw. On a failure, the creature is restrained. On a success, the creature's speed is halved as long as it remains in the cone. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. The cone lasts for 1 minute or until the breath is used again. The dragon is not affected by the cone. 

Platinum Breath. (Recharge 5–6). 

The dragon exhales radiant platinum flames in a 300-foot cone. Each creature in that area must make a DC 26 Dexterity saving throw, taking 66 (12d10) radiant damage on a failed save, or half as much damage on a successful one. Evil creatures are blinded if they fail by 5 or more.

Spellcasting. The dragon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 27). Each spell is cast at 9th level:

Bahamut always starts combat with the following two spells active: Gift of Alacrity and Fortune's Favor

At Will: Sapping Sting

Once a Day: Time Stop, Reverse Gravity, Tether Essence, Reality Break, Time Ravage, Ravenous Void

Change Shape. The aspect magically transforms into any Humanoid or Beast, while retaining his game statistics (other than his size). This transformation ends if the aspect is reduced to 0 hit points or if he uses a bonus action to end it.

Intent: Maddening. An aura emanates from the dragon in a 120-foot radius until the end of its next turn or changes intent. Each creature that is within 120 feet of the dragon and aware of it must succeed on a DC 27 Wisdom saving throw or suffer short term madness for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself as a success. If a creature's saving throw is successful or ends for it, the creature is immune to the aura for the next 24 hours.

Intent: Menacing. The dragon emanates an aura of menace in a 120-foot radius until the end of its next turn or changes intent. The first time any enemy creature enters the aura or starts its turn there during a battle, the creature must succeed on a DC 27 Wisdom saving throw or become frightened of the dragon for 1 minute or until it takes any damage. Attack rolls against the frightened creature have advantage. A creature that saves against this Intent is immune to it for the next 24 hours.

Intent: Self Righteous. The dragon adds its Charisma modifier to all of its saving throws (not included in the written saves), until the end of its next turn or changes intent.

Shimmer: Adamantine The dragon's scales become adamantine. All critical hits against it are negated. Bahamut's attacks automatically crit against objects and structures.

Shimmer: Mithral The dragon's scales become mithral. If the dragon succeeds on a saving throw that targets only it and succeeds by 5 or more, the dragon is unaffected and the spell is reflected back at the caster as though it originated from the dragon, turning the caster into the target.

Shimmer: Time Slip The dragon halves the damage it takes from an attack made against it, provided it can see the attacker. The dragon can then immediately teleport, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see. This drops Mithral or Adamantine.

Rewrite History: 1d4+4 times a day the dragon can cast Temporal Shunt at the 9th level.

The aspect 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 aspect regains spent legendary actions at the start of his turn.

Attack. The dragon makes one Claw, Bite, Tail attack, or casts Sapping Sting.

Spell Casting (Costs 2 Actions). The dragon casts a spell.

Celestial Lances (Costs 3 Actions). The dragon conjures four enormous lances of magical force that plummet to the ground at four different points he can see within 150 feet of him and then disappear. Each creature in a 20-foot-radius, 100-foot-high cylinder centered on each point must succeed on a DC 27 Dexterity saving throw or take 24 (7d6) force damage. A creature in the area of more than one lance is affected only once.