This article's content is accurate to the most recent public game version, 0.4.8d.
"Catherine's ultimate skill. Bunnies deal extreme damage, potential backlash can knock her out."
Cost: 3 
Source: Catherine (starting skill)
Ultimative Dunkle Nova is a bunny skill that only Catherine can use. Using it deals extreme 
Sharp damage to all enemies, but carries a risk of knocking her out.
Trivia
Ultimative Dunkle Nova, like all of Catherine's moves, is made out of (surprisingly not very) garbled German, which translates to Ultimate Dark Nova. It is still hardly a "nova" though.