Nature of the Swarm. The Swarm can move into and through another creatures space without any issues. Because the swarm is not a single creature it is possible to be in the same space as the horde.

Multiattack. The swarm makes two Claw attacks against random opponents within it's reach. If someone is within the swarm itself these attacks are made at advantage. Alternatively the Swarm can make a Spew attack if there is no target within it's reach.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 8 (2d4 + 3) slashing damage and 9 (2d8) necrotic damage. 

Spew. Ranged Weapon Attack: +8 to hit, range 30 ft., 1 target. Hit: 28 (5d8+5) necrotic damage. On a critical hit the target must succeed on a DC 14 Constitution saving throw or be stunned until the end of their next turn.