Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Small goblin. The swarm can't regain hit points or gain temporary hit points. The swarm has vulnerability against any area of effect damage inflicted upon it.. 

Surround! On each of its turns, the swarm of goblins can use its bonus action to surround a creature in the swarm's space. When a creature that has been surrounded tries to move, it must succeed on a Strength (Athletics) check contested by the swarm's Strength (Athletics) check or a Dexterity (Acrobatics) check contested by the swarm's Dexterity (Acrobatics) check. On a failure, the creature can't move until the start of its next turn.

Multiattack. The swarm piles onto a creature and then makes three scimitar attacks. Alternatively, the swarm can make a pile on attack.

Scimitar. Melee Weapon Attack: +5 to hit, reach 0ft, one target in the swarm's space. Hit 16 (4d6 + 2) slashing damage or 9 (2d6 + 2) slashing damage if the swarm has half of it's hit points or fewer.

Pile on!  (Recharge 5-6) Every creature in the swarm's space must make a DC 13 Strength saving throw or take 28 (8d6) bludgeoning damage or 14 (4d6) bludgeoning damage if the swarm has half of its hit points or fewer. On a success, a creature takes half damage.