Argentum (Member of "Ouroboros")

Cooling after heating. If Argentum was hit by an attack that caused fire or lightning damage, Argentum has a weakness to cold damage for the next attack against her. 

Silver for monsters. All of Argentums physical attacks deal double damage against creatures created by or affected by a curse (e.g. vampires, werewolves, etc).

Poisonous Touch. When Argentum's physical attack hits a creature, the skin on the area hit turns slate gray. Affects the senses, temporary loss of smell and taste.

Stats from the hall of mirrors.
AC: 4
Hit Points: 100
Damage Vulnerabilities: Acid, Bludgeoning, Cold (if received fire or lightning damage before)
Damage Resistances: Piercing, Slashing
Damage Immunities: Poison, magic attacks which do not have acid, cold, fire or lightning affinity.

Stats from the living mirror.
AC: same as target
Hit Points: same as target
Damage Vulnerabilities: Acid, Bludgeoning, Cold (if received fire or lightning damage before)
Damage Resistances: Piercing, Slashing
Damage Immunities: Poison, magic attacks which do not have acid, cold, fire or lightning affinity.

Multiattack. Argentum makes two melee attacks with liquid blows.

Liquid Blows. Argentum creates a whip of liquid silver from her sleeve and uses it to attack a target. Melee Weapon Attack: +8 to hit, reach 20 ft., one creature. Hit: (1d8 + 2) slashing damage plus (2d6) poison damage.

Toxic Vapor. Argentum extends her hand toward all creatures within range (10 feet) and releases noxious gas from her palm. The creatures must succeed on a DC 16 Constitution saving throw. If this fails the target receives (2d12 +4) poison damage, and half as much on a successful roll.

Toxic Grip. Grabs a target with her hand, causing discoloration of the skin and disrupting the sense of smell and taste when hit. Melee Weapon Attack: +5 to hit, reach 3 ft., one target. Hit: (2d12) poison damage. In addition, the target is considered poisoned. It takes (1d6) poison damage at the start of its turn for the next 3 turns.

Magic mirror. When Argentum is attacked with magic, she can create a mirror of silver in front of her, which intercepts the attack and bounces it back at the attacker.
Cannot be used when using a magic that deals Acid damage. May overheat and fail with repeated use.

Argentum 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. She regains spent legendary actions at the start of her turn.

Liquid Blows. Argentum creates a whip of liquid silver from her sleeve and uses it to attack a target. Melee Weapon Attack: +8 to hit, reach 20 ft., one creature. Hit: (1d8 + 2) slashing damage plus (2d6) poison damage.

Living Mirror (Costs 2 Actions). Argentum creates a clone of a medium sized being from liquid silver. The mirror clone takes the form of a being touched by Argentum in the last hour. The mirror clone takes on the outline of the person, but looks like it is made entirely of reflective silver. The mirror clone inherits the person's current stats and also uses the same weapons (at the time the person was touched by Argentum). The clone cannot use magic or special abilities that cannot be implemented physically. The clone has no consciousness of its own and follows Argentum's orders. Argentum is also able to clone herself.

Hall of mirrors (Costs 3 Actions). Argentum covers the ground with a puddle of reflective silver with herself as the center. The resulting liquid mirror on the floor has a radius of up to 15 feet. Silver stalagmites up to 7 feet tall form from the coated floor.
The area within the 15 feet radius is now considered difficult terrain to traverse. All attack rolls in the area have a disadvantage, Argentum's ability checks and attack rolls have an advantage. The mirror cabinet remains until it is destroyed or disbanded by Argentum.

If she uses the ability again, the radius is doubled. She can do this up to 120 feet. (15 ft.-->30 ft.-->60 ft.-->120 ft.)

Once the radius reaches 60 feet, all medium creatures have additional disadvantage on ability checks. Additionally, all ranged attacks from outside the mirror cabinet that follow a straight line are ineffective against all creatures inside. If magic is used to attack inside, that attack is reflected and shoots wildly through the area. Each creature inside rolls a Dexterity saving throw, the creature with the worst roll is hit by the attack.