Circle of Pestilence

Druids of the Circle of Pestilence embrace the natural consumption of life by nature. Considered evil by most, Pestilence druids abhor intervention in what they consider to be the natural cycle of life and death. Confident that life will always find a way to flourish, they dedicate themselves to preventing the natural cycle from stagnating. 

Swarmshape

At second level, you may you may wildshape into a swarm of insects (and it's variants) (MM P 338). You may add your profeciency bonus to attacks made while in swarmshape. For each level beyond 2nd, add 5 hit points to the swarm's maximum hit point value while swarmshaped. 

Circle Spells

Your knowledge of decay and pestilence infuses you with the ability to cast certain spells. At 3rd, 5th, 7th, and 9th level you gain access to circle spells connected to disease and death. 

Once you gain access to a circle spell, you always have it prepared, and it doesn’t count against the number of spells you can prepare each day. If you gain access to a spell that doesn’t appear on the druid spell list, the spell is nonetheless a druid spell for you.

Caress of Decay

At 6th level, when you damage a creature with a melee attack or a melee spell attack, you may add an additional 1d8 necrotic damage. At levels 11 and 17, add an extra 1d8 to the damage done. 

Corrupt Conditioning

At 6th level, your constant exposure to the purgative forces of nature grants you resistance to necrotic and poison damage. 

Feast of Filth

At 10th level, while swarmshaped you can feed off corpses. Feeding requires you to use your action and reduce your speed to 0 for one round. You regain 1d8 + your proficiency bonus of hit points for every round you spend feeding. Tiny creatures are entirely consumed after 1 round, small creatures provide 2 rounds of feeding, and larger creatures provide exponentially more food per size category (following the progression 2^n = 1, 2, 4, 8, 16, 32 for tiny, small, medium, large, huge, and gargantuan. 

Putrid Palate

At 10th level, you cannot become poisoned (as per the "poisoned" condition) from eating rotten food. 

One with Disease

At 14th level, you become immune to all magical and nonmagical diseases. 

Contagious Aura

At 14th level, any living creature within 5 feet of you must make a constitution saving throw equal to your spell save DC or take 6d10 necrotic damage, as well as become poisoned. If a creature succeeds on the save, they become immune to your aura for 24 hours. You cannot control what creatures are effected by this aura, meaning your allies are susceptible to it as well. The aura only activates when you take your first turn in combat (a surprise round initiated by yourself does trigger this effect).