Swarm of Scorned Familiars

The swarm can't be affected or detected by spells of 2nd level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects. In addition, if the swarm is subjected to a spell or other magical effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw.

When the swarm kills a creature that has a familiar, the familiar joins the swarm, causing the swarm's hit point maximum and current hit points to increase by 10.

The swarm immediately knows when a spell of 3rd-level or higher has been cast within 1000 feet of it, and it can sense the direction to the spellcaster's location.

The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny familiar.

The swarm makes two attacks.

Melee Spell Attack: +6 to hit, reach 0 ft., one target in the swarm's space. Hit: 14 (4d6) force damage plus 14 (4d6) necrotic damage, or 7 (2d6) force damage plus 7 (2d6) necrotic damage if the swarm has half its hit points or fewer.

The swarm attempts to siphone some magical power from a creature it can see within 10 feet of it. If the target has any remaining spell slots, it must make a DC 16 Saving throw using its spellcasting ability. The target makes this saving throw with disadvantage if it has less than half its spell slots remaining. On a failed save, the target loses one spell slot. If the target has no remaining spell slots of that level, it instead loses its highest remaining spell slot below that level. The swarm then regains 1d8 hit points per level of the spell slot lost.

The swarm adds 3 to its AC against one attack that would hit it, as it creates an instantaneous shield of force around itself.