Avarice, Scion of Levistus

Icy Doom. When Avarice dies, her corpse freezes for 9 days, during which time it can’t be thawed, harmed by fire, animated, or raised from the dead.

Chill of Stygia. Avarice's spells are supernaturally empowered by Levistus and ignore resistance to cold damage. 

Charm of the Adamant. Avarice's eyes are voids of utter darkness. She gains 30 temporary hit points once per day at the start of an encounter and is immune to the charmed and frightened conditions. 

Devil Sight. Avarice can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

Familial Channeling. Avarice magically channels a spell of 5th level or lower through one of her familiars or Stygian gargoyles as though it was the caster. Avarice expends a spell slot as usual and must concentrate on the spell if it has the concentration tag. If Avarice is concentrating on a spell cast through her familiar or stygian gargoyle and it is destroyed, she takes 4d6 psychic damage and the spell ends.

Special Equipment. Avarice wields a staff of frost with 10 charges (see “Actions” below) and a cursed ring of free action.

Cursed Ring. If another creature ever removes the ring of free action from Avarice, it shatters and is destroyed in a retributive blast of icy cold dealing 44 (8d10) cold damage to the creature who removed it. If this damage would reduce the target to 0 hp it becomes a frozen statue of ice (as though it failed the flesh to stone spell) for 9 days during which time it can’t be thawed or harmed by fire. The target dies if it is not restored before the end of the 9th day.

Spellcasting. Avarice is an 11th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 17; +9 to hit with spell attacks). She knows the Eldritch Blast cantrip and has the following wizard spells prepared:

Cantrips (at will): Eldritch Blast (see "Actions" below), fire bolt (see “Actions” below), mage hand, message, minor illusion, prestidigitation

1st level (4 slots): armor of Agathys, detect magic, mage armor, magic missile

2nd level (3 slots): flaming sphere, knock, misty step, scorching ray

3rd level (3 slots): bestow curse, counterspell, dispel magic, fireball, fly, lightning bolt

4th level (3 slots): banishment, fire shield, greater invisibility

5th level (3 slots): Bigby’s hand, Rary’s telepathic bond, far step, teleportation circle

6th level (1 slot): contingency, eyebite, investiture of ice, soul cage

Eldritch Blast (Cantrip). Ranged Spell Attack: +6 to hit, range 120 ft., three targets. Hit: 5 (1d10 + 2) force damage.

Fire Bolt (Cantrip). Ranged Spell Attack: +9 to hit, range 120 ft., one target. Hit: 11 (3d10) fire damage.

Flesh to Ice (Recharges after a Long Rest). Avarice channels the power of Levistus to cast flesh to stone (spell save DC 14) without requiring concentration. On a failure, the target becomes a statue of ice instead of stone which can’t be thawed or harmed by fire for 9 days. If the target is not restored by greater restoration or similar magic before the end of the 9th day, it dies.

Staff of Frost. While holding this staff, Avarice can expend 1 or more of its charges to cast one of the following spells from it (spell save DC 17): cone of cold (5 charges), fog cloud (1 charge), ice storm (4 charges), or wall of ice (4 charges). The staff regains 1d6 + 4 charges daily at dawn. If its last charge is expended, roll a d20; on a 1, the staff turns to water and is destroyed.

Far Step. Avarice expends a 5th level or higher spell slot and casts far step.

Fire Shield. Avarice expends a 4th level or higher spell slot and casts fire shield.

Misty Step. Avarice expends a 2nd level or higher spell slot and casts misty step.

Banishing Rebuke (Recharges after a Long Rest). When Avarice is damaged by a creature that she can see within 60 feet of her, she can banish that creature to a frigid extradimensional prison for 1 minute. While there, the creature is incapacitated and takes 5 (1d10) cold damage at the start of each of its turns. At the end of each of its turns, the creature can make a DC 14 Charisma saving throw, escaping the prison on a success and reappearing in the space it left or in the nearest unoccupied space if that space is occupied. A creature that dies in the prison is trapped there indefinitely.

Counterspell. Avarice expends a 3rd level or higher spell slot and casts counterspell.

Soul Cage. Avarice expends a 6th level spell slot and casts soul cage.

Avarice has 3 legendary actions which she can use only at the end of another creature's turn. She regains all expended legendary actions at the start of each of her turns.

Cast a spell. (Costs 1, 2, or 3 actions) Avarice expends a spell slot to cast a spell of 3rd level or lower which costs a number of legendary actions equal to its level (cantrips cost 1 action).

Lethargic Chill. Avarice targets a point she can see within 30 feet of her and lets loose a blast of ice in a 15 ft radius. Affected creatures in the radius that lack immunity to cold must make a DC 17 Dexterity saving throw or take 4d6 cold damage, have their speed reduced by 10 feet, and they can't take reactions until the start of their next turn. On a success, a creature takes half damage and suffers none of the additional effects. 

Infernal Escape (Costs 2 actions). Avarice magically ends one spell affecting her and moves up to her speed without provoking opportunity attacks. If she is concentrating on far step she can teleport up to 60 feet instead of using her movement speed.

Minions of Levistus (Costs 3 Actions). Avarice expends a 5th level spell slot and magically summons two Stygian gargoyles from the frozen wastes of Stygia to unoccupied spaces within 30 feet of her.