Sorry to be extremely late on this topic but I actually think the "powerfull" could be intentional, seeing that it's an archaic spelling.* Then again you might have wanted the whole phrase to have a more Middle English feel but that would of course also have made it quite unintelligible to most, while this "powerfull" is still comprehensible but still evokes a by-gone age.
Of course someone might just have messed up, but, hey, I'm a medievalist, I think these things are fun *Mandatory examples: "My pure powerfull gods I prestly pauoure." (The Wars of Alexander, ca 1450)
"These workers of euyll..cast be they downe: by thy powerfull hand." (Psalter, 1567)
"Whose top-branch..kept low Shrubs from Winters pow'rfull Winde." (Shakespeare)
"Hinc satis elucet maiorem habere uim ad discenda ista liberam curiositatem quam meticulosam necessitatem.”
- Augustinus Hipponensis