Friendship is undoubtedly the greatest magical power existent within the land of Equestria.
However, even something held ideal and pure as friendship can still cast a shadow.

Fear, is that shadow.

In an alternate tale, what would happen should Tirek have not faced the goodness of friendship, and instead faced the crippling darkness and power which is fear? Who better to teach Tirek what true darkness is, what feeble use of power the Centaur had in comparison to his, and how easily it can be shattered; than the King who had never allowed Tartarus to hold him, not even in death?