Circle of Corruption

Druids of the Circle of Corruption don't limit themselves to the power that nature can grant them. May it be driven by curiosity, or encouraged by a particularly hostile environment, those druids have exposed themselves to the more unnatural creatures that thrives in the wilderness. Their instincts are feral and they often prefer to solve their problem aggresively.
However, some druids of that circle invested their energy to understand and relate with the monsters that are generally chased for their weirdness and unwelcoming appearences. That weirdness is embraced by those who focus on imitating these various creatures, which makes them move flexible shapeshifters, despite the fact that delving into those surnatural powers is often taxing for the user.

Bonus Cantrip

When you choose this circle at 2nd level, you learn the Primal Savagery cantrip. If you already know this cantrip, you can learn any other cantrip from the druid's spell list.

Surnatural Wild Shape

The stranges rites from this circle grant you the ability to use your Wild Shapes beyond normal considerations. Starting at 2nd level, when you use Wild Shape you can assume the form or either a Beast or a Monstrosity that you have already seen.
    Additionally, the max challenge rating is doubled for you when you use your Wild Shape to transform into a Beast or Monstrosity. You must abide by the other limitations.

Surnatural Defense

Starting at 2nd level, while you are transformed by Wild Shape, you can expand a spell slot as a reaction when you take damage from a creature you can see within 5 feet. The creature then takes 1d10 acid damage per level of the expanded spell slot, as vengeful magical spikes emerge from your body to leash out at the target in retribution. When you reach level 6, the range increases by 5 feet (10ft.), then by another 10 feet at level 10 (20ft), and at level 14 (30ft).

Monstrous Shape

Starting at 6th level, you can use your Wild Shape feature to expand a spell slot to temporarily ignore the max CR. If you decide to do so, the max CR is instead equal to the level of the expanded spell slot. However, since this kind of transformation is very difficult to maintain and to endure, the form can only be assumed a number of minutes equal to your Wisdom modifier, and the form you assume is only at half its total hit points.
    If you transform into a creature that has Innate spellcasting, you can freely cast spells that the creature can cast at will, but must expand spell slots of the appropriate level for any other spells (even if you don't have the spell prepared). If you transform into a creature that has spellcasting, you can only use spells that you know and have prepared from their list of available spells. Casting spells this way consumes your spell slots as normal. If a creature has a spellcasting ability other than Wisdom, their spellcasting ability becomes Wisdom for you. If you transform into a creature that has weapon attacks, this means that it is possible to use weapons while in this form. However, you must still weapons that you already have on yourself.

Violent Shape

At 10th level, your attacks critically strike on a 19 or on a 20 while in your Wild Shape. Furthermore, you gain the ability to cast the Primal Savagery cantrip even while in your Wild Shape.

Monstrous Mastery

Starting at 14th level, when you use your Monstrous Shape feature to assume the form of a powerful Monstrosity, you can now spend an additional use of your Wild Shape (2 total) to add your druid level to amount of minutes that you can assume the shape and the creature is at full hit points when you transform into it.
Additionally, if you used this feature, when you revert back to your original form, you can make a Wisdom check DC 15. On a successful check, you regain one use of your Wild Shape.