Cannibal Pixie Swarm

Magic Resistance. The pixies have advantage on saving throws against spells and other magical effects.

Innate Spellcasting. The pixie’s innate spellcasting ability is Charisma (base spell save DC 12). They can innately cast the following spells, requiring only its pixie dust as a component:

At will: druidcraft

8/day each: confusion, dancing lights, detect evil and good, detect thoughts, dispel magic, entangle, phantasmal force, polymorph, sleep

For each of these spells, the 8 pixies in the swarm can work together to upcast the spell by one level for each pixie helping. Doing so increases the saving throw needed. For example:

8 pixies casting confusion: Wisdom save DC 19, AoE 35' radius

8 pixies casting polymorph: Wisdom save DC 19 (one creature), or DC 12 AoE 20' radius

The upcast drops proportionately with the HP of the swarm. When the swarm's HP drops to 8, the upcast is halved and when it drops to 1, the swarm can't upcast. 

Superior Invisibility. The pixies magically turn invisible until its concentration ends (as if concentrating on a spell). Any equipment they wear or carry are invisible with it.