This form is only possible to reach when both Buttercup and the spirits of the forest become one being to fight off incredibly powerful dangers...this form was only used a single time and she hates having to use it as it forces her to surrender control of her body to the spirits so they can basically just use her body as a vessel. You can tell she's in this form when her dark side's eyes glow...if you see the eyes glowing you better hope you aren't the target as it means you truely will not survive the encounter...

Art by  krushi.kromsai