Ancient Mist Dragon

Misty: This dragon can be in another creatures space. If a creature ends it's turn in the dragons space they take(1d12 + 4) force damage and pushed 5ft. towards the nearest empty spot for every 5ft. away they are from the nearest empty spot.

Ethereal Sight: The mist dragon can see 30ft. into the Ethereal plane at all times unless blinded.

Mere Invisibility: The dragon is extremely hard to see and can blend well in any area. The dragon has advantage on stealth checks and at the start of an enemies turn they must roll a perception check 18 or have disadvantage on any attacks made against the dragon until the dragon moves.

Multi-Attack: The dragon makes one bite attack and two claw attacks.

Bite: Melee Weapon Attack:  +13 to hit, reach 5 ft., 1 target. Hit: 32  (6d8 + 8)  force damage. 

Claw: Melee Weapon Attack:+13 to hit, reach 5 ft., 1 target. Hit: 20 (4d6  + 8) force damage.

Tail: Melee Weapon Attack:+13 to hit, reach 15 ft., 1 target. Hit: 20 (4d6  + 8) force damage.

 Misty Breath (Recharge 5–6) The dragon exhales a strong wet wind in a 50-foot cone. Any creature in this cone must make a Strength saving throw 19 or take 66 (22d6) force damage and be knocked prone and pushed 40ft. backwards. They take half as much damage, move backwards half the distance, and are not knocked prone on a success.

Misty Step: Mist Dragons can cast misty step as a bonus action.

Gaseous: As a reaction a mist dragon can chose to evaporate and reappear in a adjacent spot when they would be hit with attack. This causes the attack to be ineffective unless it was an area effect.

Ethereal Jump(3/Day): The mist dragon can jump to the Ethereal plane to avoid taking damage from a attack that had hit or that was an area effect attack. The dragon returns in any spot within 60ft. of where it jumped. It can still see on the Material plane while in the Ethereal plane.

 The mist dragon has five legendary actions which it can spend after another creature takes a turn. the dragon regains expended actions at the start of it's turn.

Tail(Cost one Action): The dragon makes a tail attack.

Teleport(Cost one Action): The dragon can teleport to any space within 90ft. of it that it can see. Creatures that had been in a 15ft. radius of the dragon must make a Constitution saving throw 16 or take (2d10) thunder damage.

Thicken Mist(Cost Two Actions): The mist that had originally granted a limited 30ft. view suddenly thicken. All creatures (except the dragon) can only see 10ft. away. As a bonus action the dragon can chose to unthicken the mist.

Heat Up(Cost Three Actions): All attacks that the dragon makes do an additional 3d6 fire damage until the end of it's turn. Misty does an additional 1d6 fire damage per 5ft.

Recharge(Cost Two Actions)(1/Day): The dragon recharges it's breath weapon. If it is already charged it uses it without spending the charge.