Lord of the Low

The Lords of the Low find their source of power in the lowliest of creatures: insects, rats, spiders, and other pests. Unmistakably, your coming is signaled by the scrambling of little claws and the cawing of the crows. You hold legions of these creatures at your command, ready to swarm, bite, and claw, and you can become one of them, to walk among your children.

Tier 1

Vermin Lord

You are a friend of the lower creatures. Before making an attack against you; a tiny beast or swarm of tiny beasts must make a Wisdom saving throw. On a failed save, the beast loses the attack. If a beast's saving throw is successful, it is immune to this feature for the next 24 hours. You also gain the ability to speak basic concepts to all forms of vermin (insects, rats, spiders, and other pests).

Might of the Small

Whenever you successful hit a creature that is larger than you with a melee attack you may deal additional damage equalt to your level. You may use this ability an number of times equal to your proficiency modifier per long rest.

Tier 2

Earth sense

Vibrations in the ground echo through its ants and earthworms, and by extension, you can sense them. You gain tremorsense with a range of 10 feet. If you already have tremorsense increase its range by 10ft.

Underfoot Combat

While you are adjacent to a creature that is two or more size categories larger than you, you can use your bonus action to enter that creature's space. You remain in this creature's space until it moves or until you use your bonus action to move to an unoccupied space adjacent to the creature. While in this space, you have half cover if the creature is two size categories larger than you, and three-quarters cover if the creature is three or more size categories larger than you.

Image credit: Ratcatcher, Magic the Gathering, Dan Scott
Subclass ideas: Vermin lord and titan slayer, Mage Hand Press