Chardansearavitriol's Ghost

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 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 the dragon's Frightful Presence for the next 24 hours.

Ethereal Sight. The ghost can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.

Incorporeal Movement. The ghost can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Paralysis: Those hit by a ghoul’s bite or claw attack must
succeed on a DC 12Con save or be paralyzed for 1d4+1
rounds. Elves have immunity to this paralysis. The save DC is
Charisma-based.

Manifestation: Every ghost has this ability. A ghost dwells
on the Ethereal Plane and, as an ethereal creature, it cannot
affect or be affected by anything in the material world. When a
ghost manifests, it partly enters the Material Plane and
becomes visible but incorporeal on the Material Plane. A manifested ghost can be harmed only by other incorporeal creatures, magic weapons, or spells, with a 50% chance to ignore
any damage from a corporeal source. A manifested ghost can
pass through solid objects at will, and its own attacks pass
through armor. A manifested ghost always moves silently.
A manifested ghost can strike with its touch attack or with a
ghost touch weapon (see Ghostly Equipment, below). A manifested ghost remains partially on the Ethereal Plane, where is it
not incorporeal. A manifested ghost can be attacked by opponents
on either the Material Plane or the Ethereal Plane. The ghost’s
incorporeality helps protect it from foes on the Material Plane, but
not from foes on the Ethereal Plane.
When a spellcasting ghost is not manifested and is on the Ethereal Plane, its spells cannot affect targets on the Material Plane,
but they work normally against ethereal targets. When a spellcasting ghost manifests, its spells continue to affect ethereal targets
and can affect targets on the Material Plane normally unless the
spells rely on touch. A manifested ghost’s touch spells don’t work
on nonethereal targets.
A ghost has two home planes, the Material Plane and the
Ethereal Plane. It is not considered extraplanar when on either
of these planes.

Telekinesis: A ghost can use telekinesis as a standard action
(caster level 12th or equal to the ghost’s HD, whichever is higher).
When a ghost uses this power, it must wait 1d4 rounds before
using it again.

Multiattack. The ghost dragon makes one claw attack and one withering bite attack.

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Withering Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 15 (2d10 + 4) piercing damage, and the target must succeed on a DC 17 Constitution saving throw or take 18 (4d8) necrotic damage.

Horrifying Breath (Recharge 5-6). Ranged spell attack. The ghost dragon exhales a blast of icy terror in a 30-foot cone. Each living creature in that area must make a DC 16 Wisdom saving throw. On a failure, a creature takes 44 (8d10) psychic damage and is frightened for 1 minute. On a success, it takes half the damage and isn’t frightened. A frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. In the empty dragon’s lair with no trace of treasure or beast, a cold breeze from nowhere wafts through the chamber. A wavering draconic form fades into sight, its hissing anger leaking out through translucent teeth.

Etherealness. The ghost enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.

Horrifying Visage. Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 × 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

Possession (Recharge 6). One humanoid that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or another effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics but doesn't gain access to the target's knowledge, class features, or proficiencies.

The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the ghost reappears in an unoccupied space within 5 feet of the body. The target is immune to this ghost's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

Reactions

 50% chance to ignore damage from a corporeal source. 

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.

Possess Dragon (Recharge 5-6). One dragon that the ghost dragon can see within 10 feet of it must succeed on a DC 16 Charisma saving throw or be possessed by the ghost dragon. The ghost dragon then disappears, and the target is incapacitated and loses control of its body. The ghost dragon now controls the body but doesn’t deprive the target of awareness. The ghost dragon can’t be targeted by any attack, spell, or other effect except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target’s statistics but doesn’t gain access to the target’s knowledge, class features, or proficiencies. The possession lasts until the body drops to 0 hit points, the ghost dragon ends it as a bonus action, or the ghost is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the ghost dragon reappears in an unoccupied space within 10 feet of the body. The target is immune to this ghost dragon’s Possess Dragon for 24 hours after succeeding on the saving throw or after the possession ends. The ghost dragon loses its Horrifying Breath attack. Alternatively, the recharge for this action and for Horrifying Breath can be modified to recharge on 6.
Shade of Life Breath (Recharge 5-6). The ghost dragon exhales a blast of energy reminiscent of its former existence combined with the terrible energy powering its current existence. Each living creature in the ghost dragon’s 30-foot-cone blast must make a DC 17 Reflex saving throw, taking 27 (5d10) damage based on the living dragon’s breath (acid, cold, fire, and so on, but the damage type doesn’t change) and 27 (5d10) psychic damage on a failed save or half as much damage on a successful one. A living dragon that fails its saving throw must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. The ghost dragon loses its Horrifying Breath attack.

Descendant of the Gods (Mythic Trait: Recharges after a short or long rest) When the ancient Dragon is reduced to 0 hp, it doesn't die or fall unconscious. Instead,  it is filled with device energy coming from the dragon gods (Bahamut for metallic, Tiamat for chromatic), regaingin hit points equal to its maximum HP. 

Mythic Actions. 

If the Ancient Dragon's mythic trait is active, it can use the options below as legendary actions for 1 hour after using the Descendant of the Gods.

Show of Force. The Ancent Dragon makes a bite and a tail attack. 

Ethereal. (2 actions) The Ancient dragon moves through all solid objects up to 120ft to an unoccupied space it can see. Does not provoke an attack of opportunity. 

Draconic determination (3 actions).  The ancient dragon recharges its Breath Weapon and uses it.