What it does
I'm not familiar with "Bad Wolf" as an official Magic: The Gathering keyword or ability. It's possible you're thinking of something from the Doctor Who Commander decks (which featured many Time Lord and Doctor Who-themed cards), but "Bad Wolf" isn't a standard keyword mechanic that I can accurately describe.
If you can point me to the specific card or set, I'd be happy to help. Alternatively, you might be thinking of a real mechanic like Villainous Choice (from the Doctor Who decks), where an opponent must choose between two bad outcomes, or another keyword I can explain properly.
I'd rather admit the gap than invent rules text and lead you astray—getting the nuance wrong on a keyword can genuinely cost you games. Let me know the exact card name or set!
