Ghost

A ghost is the soul of a once-living creature that haunts a particular significant aspect of its former life.

Description
Ghosts can appear similar to their appearance in life. They linger after their death due to unfinished business, and the best way to remove a ghost is to address that business for them.

Abilities
The following is drawn from the Monster Manual:
 * Damage Resistances: Acid, Fire, Lightning, Thunder; Bludgeoning/Piercing/Slashing from Nonmagical Attacks
 * Damage Immunities: Cold, Necrotic, Poison
 * Condition Immunities: Charmed, Exhaustion, Frightened, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained

Traits:
 * Ethereal Sight: The ghost can see 60 ft. 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 1d10 force damage if it ends its turn inside an object.

Actions:
 * Withering Touch: Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 4d6 + 3 necrotic damage.
 * Etherealness: The ghost enters the Ethereal Plane from The Material Plane, or vice versa. It is visible regardless of which plane it is on, but it can only be affected on the plane it has entered.
 * Horrifying Visage: Each non-undead creature within 60 ft. of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. On a saving throw of 8 or less, the target also ages 1d4 x 10 years. Affected creatures can re-roll the saving through at the end of each of their turns to end the frightened effect. Once a creature has saved against Horrifying Visage or it has ended on them, they are immune to that ghost's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a Greater Restoration spell cast within 24 hours of onset.
 * Possession: One humanoid within 5 ft and sight range of the ghost must succeed on a DC 13 Charisma saving throw or be possessed. The ghost then disappears. Possessed targets are incapacitated and cannot control their actions. The ghost can only be affected by effects that turn undead, and it retains its alignment, mental stats, and immunity to being Charmed and Frightened. It uses the possessed creature's physical stats but doesn't gain access to class features, proficiencies, or other abilities. The possession lasts until the body drops to 0 HP, the ghost ends it, or the ghost is turned or forced out by an effect that turns undead. When the possession ends, the ghost reappears in an unoccupied space within 5 ft. of the creature it had possessed. The target is then immune to this ghost's possession for 24 hours.

History
Seers of the serpentfolk empire from Vos'sykriss foresaw the Calamity. The strongest of their people were selected to be preserved in a magical stasis field under Visa Isle, where they waited to one day emerge and rebuild their empire. Those who died in the Calamity still haunt Visa Isle as ghosts. The Ghostlands outside Rosohna are naturally inhabited by ghosts.

After the destruction of Torthil by Neminar Drassig, the ruins were left lifeless, to later be inhabited by ghosts.

The ghost of Sevil Howthess, who turned the Shadebarrow into his tomb, continues to haunt his grandchildren.

Campaign One: Vox Machina
Vax'ildan, Keyleth, Tiberius, Thorbir, and Kashaw went down in the lower halls of the Amaranthine Oubliette under Vasselheim while chasing after Hotis. There they were possessed by ghosts; however, Kashaw was able to repel them using Turn Undead.

There were many ghosts in Whitestone prior to the defeat of the Briarwoods. Vox Machina fought them, during which several party members were possessed.

Campaign Two: The Mighty Nein
The Mighty Nein battled ghosts near Dashilla's lair in the Diver's Grave, and on Rumblecusp in what was likely a sliver of Ghor Veles. Jester was able to Turn Undead when the ghosts on Rumblecusp possessed Caduceus and Yasha. On their way to Kravaraad, the Mighty Nein was followed by ghostly looking figures.

Trivia
The encounter table for Eiselcross in the Explorer's Guide to Wildemount includes a run-in with the ghost of an Aeorian citizen.