Black Glass Leviathan, Vassal of Hatred

Nightmarish. Creatures frightened by the Vassal have disadvantage on saving throws against its effects.

Immutable Form. The Vassal is immune to any spell or effect that would alter its form.

Magic Resistance. The Vassal has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Vassal's weapon attacks are magical.

Legendary Resistance (3/Day). When the Vassal would fail a saving throw, it can choose to succeed instead.

Earth Glide. The Vassal can burrow through nonmagical, unworked earth, stone, and glass. While doing so, the Vassal doesn't disturb the material it moves through.

Multiattack. The Vassal can make three Dreadful Glare attacks. It can also use its Unspeakable Horror, if available.

Bite. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 32 (4d12 + 8) piercing damage. 

Dreadful Glare. The Vassal glares at a creature within 120 feet of it. Roll 1d6 for its effect. If the effect requires a saving throw, the DC is 19. If the effect requires an attack roll, it has a +12 to hit.

1. Furious Glare. The creature must make a Charisma saving throw. On a failed save, the creature must immediately make an attack against the nearest creature (excluding the vassal). It may use spells or ranged attacks for this, but the Vassal cannot cause it to expend a spell slot higher than 4th level. On a successful save, the target take 20 (3d12) psychic damage.

2. Terrifying Glare. The creature must make a Wisdom saving throw. On a failed save, the creature is Frightened of the Vassal. The creature can repeat the save at the end of each of its turns, ending the effect on itself on a success. If the target was already frightened by the Vassal, it is also paralyzed while it is frightened. If it was already paralyzed, the creature is then petrified until cured. On a successful save, the creature takes 20 (3d12) poison damage. 

3. Darkfire Glare. The creature must make a Dexterity saving throw. On a failed save, the target takes 26 (4d12) fire damage and 26 (4d12) cold damage, or half on a success.

4. Draining Glare. The creature must make a Strength saving throw. On a failed save, the creature is weakened and is knocked prone. While weakened, the creature's movement is halved, it has disadvantage on attack rolls and ability checks using strength, and it does half damage with strength-based attacks. The creature can repeat the save at the end of each of its turns, ending the effect on itself on a success. On a successful saving throw, the creature instead takes 20 (3d12) necrotic damage.

5. Shattering Glare. The creature must make an Intelligence saving throw. On a failed saving throw, the creature is mindbroken. While mindbroken in this way, the creature has disadvantage on Intelligence, Wisdom, and Charisma checks and attacks. Additionally, whenever a creature would cast a spell, first roll a concentration check, the DC of which is 2 x the spell's level. On a failed save, the creature cannot cast the spell (neither the slot nor the action are wasted). On a success, the spell is cast as is normal. A creature can repeat this saving throw at the end of each of their turns, ending the effect on themself as a success. A creature who succeeds the saving throw instead takes 20 (3d12) force damage.

6. Entropic Glare. The creature must make a Constitution saving throw. On a failed save, it falls unconscious for 1 minute. It can repeat the saving throw whenever the creature takes damage, another creature uses an action to attempt to rouse them, and at the end of each of their turns. On a successful saving throw, the creature instead takes 20 (3d12) points of cold damage.

Unspeakable Horror (5-6). Each creature within a 120 foot radius of the Vassal that can see it must make a Wisdom saving throw (DC 17). On a failed save, the creature is pushed 15 feet away, knocked prone, frightened for 1 minute, and takes 26 (4d12) points of necrotic damage. The creature may attempt to save at the end of each of its turns, ending the effect on itself on a success. On a successful save, the creature takes half damage and is not frightened or knocked prone.

The Vassal can take 3 legendary actions, choosing from the options below. It can take only one legendary action at a time and only at the end of another creature’s turn. The Vassal regains spent legendary actions at the start of its turn.

Move (Costs 1 Action). The Vassal moves up to 120 feet.

Glare (Costs 1 Action). The Vassal uses a single Dreadful Glare of its choice.

Inspire Hatred (Costs 2 Actions). One creature of the Vassal's choice within 120 feet of it must make a Charisma saving throw (DC 17). On a failed save, the Vassal becomes invisible to the creature, and an illusory duplicate visible only to the creature appears in the space of its closest ally. Any attacks directed at the Vassal by this creature target the ally instead. This effect persists until the end of the creature's next turn.