Swarm Patron

The swarming masses of things that creep and crawl may be unintelligent individually, but each one is a single brain cell in the collective hivemind that is the Will of the Swarm. The Swarm's only goal is to feed and multiply, and to achieve that goal, they sometimes incorporate especially receptive Scuttling Trees (humanoids) into their hivemind, granting them the power to command the swarm in exchange for finding new prey to feast upon. Those that serve the Swarm well can even wholly become part of it, relinquishing their body as food but keeping their consciousness in tact as one of the dominant Wills within the Swarm. 

3rd Level - Hivemind Implant

The Swarm Patron Warlock becomes a part of their patron's hivemind. They gain the ability to communicate telepathically with insects and arachnids within 60 feet. These communications are in the form of mental images and single word instructions.

Gain access to the Infestation cantrip, and it can target one additional creature.

6th Level - Cloud of Flies

The Swarm Patron Warlock has learned to better control their command over the swarming masses. Gain permanent access to the Eldritch Invocation Cloak of Flies.

While the Cloak of Flies is active, the range is increased to 10 feet, it's damage is now twice the Warlock's Charisma modifier, and they can now choose any number of creatures to not be damaged by it.

10th Level - Swarm Dominance

While Cloak of Flies is active, the Warlock reduces all damage taken by 3.

Gain resistance to Poison and Acid damage.

When dealing Poison or Acid damage, deal an additional 2 damage per damage die.

The spell Insect Plague can deal Poison or Acid damage instead of Piercing damage.

14th Level - The Swarm that Walks

The Warlock has achieved true communion with the Swarm, giving their body as food to become one with it. Cloak of Flies is always active, the radius is increased to 20 feet and the Warlock's body transforms into a swarming mass of insects in the vague shape of a person. They gain resistance to Slashing, Piercing and Bludgeoning damage, are able to pass through and occupy the same space as other creatures, and the damage of the Cloak is doubled.

If they were to die, instead the insects making up their body disperse, and will recombine an hour later with half the Warlock's health.