Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Wretched. The swarm can't regain hit points or gain temporary hit points.

Wretched Pack Tactics. The Wretched Swarm has advantage on all attack rolls as long as its above half hit points, otherwise it has disadvantage on attack rolls

Bite.. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 30 (5d10+6) piercing damage, or 12 (2d10+1 ) piercing damage if the swarm has half of its hit points or fewer. on hit 2 (1d4+1) wretched attach to the target, while attached subtract 6 (1d10+1) from the damage of attacks but at the start of each turn the target takes 6 (1d10+1) necrotic damage for each wretched attached. 

The attached wretcheds move with the target whenever the target moves, requiring none of the Wretcheds movement. Each wretched can detach from the target and move back into the swarm using 5ft of movement, A creature, including the target, can use its action to detach up to 2 wretched