Circle of the Abyss

Some savage druids believe that the Abyss represents the true spirit of the wild - a testing ground where the strong devour the weak.  They turn into particularly vicious forms and tear apart their foes with their fangs and claws.

Brutal Wild Shape

When you choose this circle at 2nd level, you gain the ability to use Wild Shape on your turn as a bonus action, rather than as an action.

Additionally, while you are transformed by Wild Shape, you can use a bonus action to make your next attack that hits before the end of your next turn deal an additional 1d8 damage per level of the spell slot expended.

Demonic Circle Forms

The rites of your circle grant you the ability to transform into more lethal forms.  Starting at 2nd level, you can use your Wild Shape to transform into a beast with a challenge rating as high as 1.  You ignore the Max. CR column of the Beast Shapes table, but must abide by the other limitations there.

Starting at 6th level, you can transform into a beast with a challenge rating as high as your druid level divided by 3, rounded down.

When you use Wild Shape, the creature you turn into resembles a Material Plane beast, but is infused with the power of the Abyss.  For example, it may have glowing red eyes, spines covering its body, or an extra set of fangs.  Although you use the statistics of beasts, your type is fiend when you use Wild Shape.

You also learn the Abyssal language. 

Abyssal Strike

Starting at 6th level, your attacks in aberration form count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Demon Form

At 10th level, you can expend two uses of Wild Shape at the same time to transform into any demon with a CR of 5 or lower except a quasit, shadow demon, or dybbuk, as those demons are overly subtle.  If you transform into a tanarukk, you lack a greatsword.  Instead, you can make two claw attacks that deal 1d6 + 4 slashing damage.

Demon Summoner

At 10th level, you add the spells Summon Lesser Demons and Summon Greater Demon to your druid spell list.

When you cast Conjure Animals or Conjure Woodland Beings, the creatures you summon are considered demons rather than beasts or fey.

Bloodlust

At 14th level, your attacks rival those of demons in their ferocity.  When you score a critical hit with a Wild Shape form's attack, it deals additional damage equal to your druid level.