Like many druids, they have a strong connection to nature. Unlike many, they however accept and encourage evil life, to bring balance to the ways of  the world. If all evil was vanquished, then entire species of creatures will be gone as well. Evil needs protectors as much as good.

Affinity for Fiends

When you choose this circle at 2nd level, you can either gain the cantrip eldritch blast or chill touch.

Trait of Lemure

Starting at 2nd level, you can use the abilities of a lemure for one turn. The ability to see in the dark, resistances or immunities, or even listening to the speech of infernal.

This can be used once after a long rest.

Fiendish Invocations

Starting at 3rd level, you can learn a warlock's eldritch invocation. You may pick from: Agonizing Blast, Beast Speech, Eldritch Spear, or Fiendish Vigor.

At 8th level, you can learn one more, and the list expands to include: Gaze of Two Minds, Minions of Chaos, One with Shadows, and Sculptor of Flesh.

Manifest Nightmare

Starting at 6th level, you can summon a steed called a nightmare from someones thoughts. It will reflect them, and gain some kind of boost or negative effect compared to what this would normally summon. Only you can ride on the nightmare, other things will pass through. They will typically be summoned unable to fight. They have a flight speed of 20 feet. They gain one language that you know, and can commune telepathically.

The creature will disappear in 1 hour, and can be summoned once after a long rest.

Fiend's Force

When you reach 10th level, you have advantage on rolls that have to do with resisting fire spells, and attacking with fire.

Balors Blaze

When you reach 14th level, you gain the gift of a balor's vitality.

Once after a short rest, you can absorb any amount of flames that aren't directed as an attack to heal (Druid Level)d4 HP.