Re-roll. When the Feyton swarm fails a Dexterity saving throw they can choose to roll again once every short rest.

Spellcasting. The Feyton swarm is a 4th-level spellcaster. Its spellcasting ability is +6 (spell save DC 15, +7 to hit with spell attacks). The Feyton swarm has following 4th level spells prepared:
Cantrips (at will): Minor illusion
1st level (3 slots): Produce flame 1d6 fire damage                                                                    2nd level (2 slots):Levitate, Gust of wind                                                                                      3rd level (2 slots): Dispell magic, Fireball 3d6 fire damage                                                        4th level (1 slot): Watery sphere

Swarm. Melee attack: +7 to hit, reach 15ft., all targets within a ten foot radius of each other. Hit (3d12+5) Bludgening damage.

Whenever an opponent manages to land a hit on the Feyton swarm it gets one swarm attack.