Rampage. When the gnoll reduces a creature to 0 hit points with a melee attack on its turn, the gnoll can take a bonus action to move up to half its speed and make a bite attack.

magic resistence: adv on spell saves

can cast the following spells 

at will detect magic, disguide self

8 times per day: counterspell, dimension door, darkness, crown of madness, arms of hadar (5d6) and fly and fireball/lightnening bolt (10d6), inflict wounds (8d10)

Multiattack. The gnoll makes two attacks, either with its glaive or its longbow or bite, and uses its Incite Rampage if it can.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 5 (4d4 + 8) piercing damage.

Glaive. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 8 (2d10 + 8) slashing damage.

Longbow. Ranged Weapon Attack: +7 to hit, range 150/600 ft., one target. Hit: 6 (2d8 + 3) piercing damage.

Incite Rampage (Recharge 5–6). three creature the gnoll can see within 30 feet of it can use its reaction to make a melee attack if it can hear the gnoll and has the Rampage trait.

casting spell  - 2 action

glaive attack/longbow/bite - 1 action

try to regain/use incite rampage - 1 action

maddening burst - 3 actions
all creatures within 45 feet need to succeed a DC 20 wisdom saving throw or become insane for 1d20 * 6 seconds while insane the creature becomes prone, movement is halved and has a 2/4 chance of attacking whatever the closest creature is rather then selecting what creature they attack (dm rolls after they choose the action to prevent meta gaming) and have disadvantage on attacks against the creature who afflicted them with madness.