Comments from the Finger: Requested a few times, and I do love an evil druid.
Circle of Vermin
Druids of the Circle of Vermin, sometimes known as Vermin Lords, 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.
Lord of the Low
Starting at 2nd level, you can Wild Shape into a Swarm of Rats, Bats, or Ravens as a bonus action. At 6th level, you can also Wild Shape into a Swarm of Insects (including all Variant Insect Swarms), and at 10th level, into a Swarm of Snakes.
Also, 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.
At 2nd level, as you walk, vermin congregate at your side, and you are never to be found without a rat or spider making its nest in your robes. When you take as short or long rest, you can call a companion swarm to your aid, summoning a new swarm that you can Wild Shape into or restoring your current swarm to full hit points.
Add your proficiency bonus to the swarm’s AC and attack rolls, as well as to any saving throws and skills it is proficient in. Its hit point maximum equals its normal maximum or four times your druid level, whichever is higher.
The swarm obeys your commands as best as it can. It takes its turn on your initiative, though it doesn’t take an action unless you command it to. On your turn, you can verbally command the swarm where to move (no action required by you). You can use your bonus action to verbally command it to take the Attack, Dash, Disengage, or Dodge action.
By 6th level, 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.
Body of Plague
By 10th level, you have been in such close proximity to vectors of plague that you are immune to disease and being poisoned.
Beginning at 14th level, if you Wild Shape into the same form as your companion swarm, you can merge into a single Great Swarm as a bonus action on your turn. This new form is one size category larger than the normal swarm. While merged, your companion swarm does not act other than as part of the Great Swarm.
You have advantage on attack rolls against creatures caught in the swarm and you deal 10d10 piercing damage on a hit. If a creature attacks the Great Swarm, it has a 50% of targeting you and a 50% chance of targeting your companion swarm. If the Great Swarm takes damage from a source other than an attack, both you and your companion each take half damage. You can dissipate into two swarms as a bonus action.
Changelog: 2/23/16: Earth Sense: Changed from blindsight to tremorsense.
Empathy of the Low: Renamed Lord of the Low: Added wildshape options early.
Amass Vermin: You can summon any swarm that you can wildshape into. Don't add your proficiency bonus to the swarm's damage rolls. HP is higher of 4X druid level or swarm's original HP. Bonus action to command it to act.
We Swarm: No longer grants wildshape (now granted by Lord of the Low). Great Swarm damage set at 10d10 piercing.
2/24/16: Lord of the Low: You can assume Insect Swarm Variant forms, as provided in the sidebar on MM338.
2/27/16: Lord of the Low: Wildshape to vermin is a bonus action