Evasion. If the swarm is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Heat Aversion. When the swarm starts its turn within 10 feet of a flame or an object that takes up at least a 5-foot cube and is hot enough to cause damage, it takes 1d8 fire damage.

Snowy Camouflage. The snowball has advantage on Dexterity (Stealth) checks made to hide in snowy terrain.

Suicidal Slam. The swarm must move at least 10 feet toward its target to damage it with its slam attack. The swarm takes the same amount of damage that is dealt to its target by its slam.

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 snowball. The swarm can't regain hit points or gain temporary hit points.

Variant: Yellow snowballs. When the swarm deals damage to a creature, that creature must make a Constitution saving throw (DC 9) or become poisoned until the end of its next turn.

Slams. Melee Weapon Attack: +6 to hit, reach 5ft., one creature in the swarm's space.
Hit: 5 (2d4) bludgeoning damage plus 9 (2d8) cold damage, or 2 (1d4) bludgeoning damage and 4 (1d8) cold damage if the swarm has half of its hit points or fewer.

Snowportation. If the swarm is touching nonmagical snow, it can teleport to any other space that contains nonmagical snow that it can see as a bonus action on its turn.