Bound Haunting. Mara's spirit is bound to her domain of dread, The Gallery of the Damned. At the start of her turn, if she is outside the Gallery's walls and not possessing a creature of her bloodline using her Possession action, she must succeed on a DC 15 Charisma saving throw or vanish and reappear in an unoccupied space within the Gallery. If she dies, the Gallery is no longer her prison and characters are free to leave.
Incorporeal Movement. Mara can move through other creatures and objects as if they were difficult terrain. She takes 10 (2d10) force damage if she ends her turn inside an object.
Legendary Resistance (1/encounter). If Mara fails a saving throw, she can choose to succeed instead.
Otherworldly Perception. Mara can sense the presence of any creature within 30 feet of it that is invisible or on the Ethereal Plane. It can pinpoint such a creature that is moving.
Ruthless. Mara has advantage on melee attack rolls against any creature that has used the Help action, restored hit points to another creature, or aided another creature through a feature, such as Bardic Inspiration, or magic, such as the longstrider or aid spells, within the last 1 minute. This trait doesn’t give Mara advantage on melee attack rolls against creatures who have features or traits that passively aid others, such as a paladin’s Aura of Protection or a wolf’s Pack Tactics.
Unusual Nature. Mara doesn't require air, food, drink, or sleep.
If Mara is reduced to 0 hit points, she doesn't die or fall unconscious. Instead, lets out a terrible wail (DC 15 WIS save vs the Frightened condition in a 30 foot radius; on a fail, Frightened for 1 minute. May repeat save at the end of its turn). Any remaining portraits in the gallery become Art Elemental Mascots. Mara regains 135 hit points, becomes invisible, and moves up to twice her speed without provoking opportunity attacks.
Multiattack. Mara makes two attacks.
Enfeebling Touch. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 14 (4d6) necrotic damage, and the target’s Strength, Intelligence, or Wisdom score (DM's choice) is reduced by 1d4. The target dies if this reduces its Strength to 0. The target is incapacitated if this reduces its Intelligence or Wisdom to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
PAIN-tbrush. Ranged Spell Attack. +7 to hit, range 120 feet, one creature. Hit: 13 (2d8 + 4) necrotic damage. The PAIN-tbrush may have an additional effect if Mara uses her Palette Swap bonus action.
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 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 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.
Palette Swap. Mara dips her PAIN-tbrush into an available canvas (she must have portraits available to use this bonus action) and imbues her ranged spell attacks with a secondary effect until the start of her next turn.
Legendary Action Uses: 2. Immediately after another creature's turn, Mara can expend a use to take one of the following actions. Mara regains all expended uses at the start of each of her turns.
Harrow. Wisdom Saving Throw: DC 15, one creature Mara can see within 90 feet of herself. Failure: 11 (3d6) Psychic damage, and the target has the Frightened condition for 1 minute. The target may repeat the save at the end of each of its turns, ending the effect on a success. On a successful saving throw, the target takes half damage and is not frightened. Mara can't take this action again until the start of her next turn.
Command Portrait. Mara commands a portrait to take one of the following actions:
On Initiative count 20 (losing initiative ties), Mara can conjure another portrait into the gallery.