Limited Spell Immunity.

The swarm automatically succeeds on saving throws against spells of 3rd level or lower, and the attack rolls of such spells always miss it.

Swarm.

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

Actions

Bites.

Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm's space. Hit: 12 (3d6 + 2) piercing damage, or 5 (1d6 + 2) piercing damage if the swarm has half of its hit points or fewer, plus 7 (2d6) force damage.

Reactions

Spell Redirection.

In response to a spell attack roll missing the swarm, the swarm causes that spell to hit another creature of its choice within 30 feet of it that it can see.

Swarm of Gremishkas