Flyby. The Lamprey doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Made of Voidstuff. The Lamprey's otherworldly constitution is such that it cannot be detected by any spell or effect.

Innate Spellcasting (Psionics). The Abyssal Lamprey's innate spellcasting ability is Intelligence (spell save DC 15). It can innately cast the following spells, requiring no components:

At will: detect thoughts, invisibility

Bite. Melee Weapon Attack: 1d20+7 to hit, reach 5 ft., one target. Hit: 8 (1d4+5) slashing damage plus 7 (3d4) psychic damage. The Abyssal lamprey heals for the psychic damage inflicted. 

Synaptic Overload (Recharge 4-6); the Abyssal Lamprey reaches into the mind of one creature it can see within 30 ft and attempt to interfere with its neural processes. the creature must succeed on a DC 15 intelligence save or take 14 (4d6) psychic damage, taking half as much on a success. A creature that fails its save by 5 or much is additionally stunned until the end of its next turn.