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Flyby. The Fell Beast doesn't provoke opportunity attacks when it flies out of an enemy's reach.
Group Mentality. When the Fell Beast attacks a creature, it gains a +1 to attack rolls for every Fell Beast within 5ft of its target.
Keen Sight & Smell. The Fell Beast has advantage on Wisdom (Perception) checks that rely on sight or smell.
Death Stench. Any creature that starts its turn within 5 feet of the Fell Beast must succeed on a DC 13 Constitution saving throw or be poisoned until the start of its next turn. On a successful saving throw, the creature is immune to the Fell Beasts Stench for 24 hours.
Emboldened. As long as there is one additional Fell Beast in sight, all Fell Beasts gain a +1 to attack rolls.
If five or more are present, this increases to +3.
If ten or more are present, this increases to +5.
Bite. Melee Weapon Attack: +3 to hit, reach 5ft., One Target: hit 1d6+2 piercing damage plus 1d8 necrotic necrotic damage.
Claw. Melee Weapon Attack: +3 to hit, reach 10ft., One Target: hit 1d6+3 slashing damage.
Call for Swarm (Recharge 4-6). The Fell Beasts constant squeaking screams have a chance of attracting others, attracted to the commotion in search of food. Rolls d4, on a 4 a single Fell Beast will appear.
Fell-Gel (1/Day). Ranged Weapon Attack, 20/60ft., One Target: hit, 1d8 necrotic, and target must make a DC 17 Constitution saving throw. On a fail, the creature receives an additional 1d4 poison damage, and makes attacks with disadvantage until the gel is wiped away using an action. The creature can make another saving throw on its next turn, or receive the same effect.
On a success, the creature receives no penalty and can wipe the gel away with a bonus action.
In addition to this, the fell-gel is highly combustible, exploding upon contact with any source of fire and igniting whatever it is currently attached to, dealing 3d6 fire damage initially and and replacing all previous gel effects with 1d6 fire damage.