The bold part is the one I don't really understand, the rest is included for context. What function is "と" serving here?
This と itself is a plain old quotative particle. The part before this と is the content of the evil wizard's 企み.
... (who) protected Princess from the clutches of the evil wizard who attempts to destroy this planet
In case you've missed it, this 滅ぼさん is an archaic version of 滅ぼそう, or "I will destroy."