Mount. If the specter isn't mounted, it can use a bonus action to magically teleport onto its lesser gloamwing mount, provided the specter and the lesser gloamwing are on the same plane of existence. When it teleports, the specter appears astride the lesser gloamwing along with any equipment it is wearing or carrying. While mounted and not incapacitated, the specter can't be surprised, and both it and its mount gain advantage on Dexterity saving throws.

Multiattack. The specter makes two glaive attacks.

Glaive. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 7 (1d10 + 2) slashing damage + 9 (2d8) psychic damage. 

Whispering Keen (Recharge 4-6). The specter unleashes a swath of hideous whispers and screeches around it. Each non-Undead within 120 feet of the specter that can hear it must succeed on a DC 14 Constitution saving throw or have disadvantage on Wisdom and Constitution saving throws until the end of the specter's next turn.

Mind Twist (Recharge 5-6). The specter magically emits psychic energy in a 45-foot cone. Each creature in that area must succeed on a DC 14 Wisdom saving throw or take 19 (3d12) psychic damage and be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.