Creeping Fog

False Appearance. If the fog is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn’t observed the fog move or act, the creature must succeed on a DC 20 Intelligence (Investigation) check

Legendary Resistance (3/Day). If the fog fails a saving throw, it can choose to succeed instead.

Mist Form. The fog can enter a hostile creature's space and stop there. In addition, if air can pass through a space, the fog can do so without squeezing, and it can't pass through water.

Unusual Nature. The fog doesn’t require air, food, drink, or sleep.

Multiattack. The fog uses Soothing Lullaby and then makes three attacks.

Mist Pseudopod. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (3d10) necrotic damage, the fog regains a number of hit points equal to half the necrotic damage taken. This damage doesn’t disturb a sleeping target.

Soothing Lullaby. Each creature of the fog's choice that is within 120 ft. of the fog and can hear it must succeed on a DC 16 Wisdom saving throw or fall unconscious for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the fog's Soothing Lullaby for the next 24 hours.

Mist Generation. A 20-foot radius sphere centered on the fog is covered by mist, becoming heavily obscured for 1 minute.

Misty Impersonation (Recharge 4-6). The fog crafts mist in the image of one Medium or smaller creature it has consumed within the last week. The image appears at a spot within 60 feet of the fog and lasts for 10 minutes, or until the fog dismisses it (no action required). The image is purely visual; it isn’t accompanied by sound, smell, or other sensory effects. The fog can cause the image to move to any spot within 60 feet of it as a bonus action.

The fog 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. The fog regains spent legendary actions at the start of its turn.

Exploding Simulacrum. Every creature within 10 feet of the image created by Misty Impersonation must succeed on a DC 16 Dexterity saving throw or be blinded until the end of their next turn.
Misty Escape (2 Actions). The fog uses Fog Generation and then moves up to half its speed.
Visions of Dread (3 Actions). Shapes that resemble the fog appear from the mist, attack rolls against the fog have disadvantage until the start of its next turn.