Devilishly Speedy. The Speed Devil is extremely mischievous with its actions, It can Disengage and Dodge as a bonus action.

Momentum Movement. Whilst moving Speed Devils can move up and down walls or across them at will, It will stay on any surface whilst moving but cannot end its turn or take an action while not on the ground or else it will fall.

Magical Attacks: Speed Devils attacks are all magical

Innate Spellcasting. The Speed Devil innate spellcasting ability is Charisma  (spell save DC 15, +5 to hit with spell attacks). it can innately cast the following spells, requiring no material components:

At will 3rd Level: Shocking Grasp , Thunderwave 

2/day: Haste, Slow.

Multiattack. Speed Devils make 2 attacks: Either 2 Speed Punches or 1 Speed Punch and a Whirling Claws, if possible. Alternatively Speed Devils can you a Mach Punch. 

Speed Punch. Melee Unarmed Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (2d12 + 2) Bludgeoning damage. 

Whirling Claws. Melee Weapon Attack: +6 to hit, up to 5 targets in a 20ft square. Hit: 10 (2d8 + 2) Slashing damage.

Mach Punch (Recharge 5/6). Melee Unarmed Attack: +6 to hit, one target. The Speed Devil uses its insane momentum to deliver a devastating blow, for each 10ft the Speed Devil has moved concurrently it deals 5 (1d6 + 2) Bludgeoning damage Up to a max of 70ft (Unless Hasted - 140ft) to maximum of 35 (7d6 + 14) or 70 (14d6 + 28) while hasted.

Phase. By vibrating its body into a high movement the Speed Devils can make one attack a round phase through its body causing it to do no damage.