they say that the secret is a beast that have more power them that fox but only the perfect uchiha cant unluck that beast from anothey dimension the beast is a Phoenix
That would be hard to believe, since the bijuu are considered stronger since tengus are considered supernatural creatures and the bijuu are considered gods. Even if the theory of Uchiha Madara vs the King of Tengu is true... The king would be weaker than the kyubi because in that fanfic theory, Madara asks kyubi the power to the defeat the king of tengus. The kyubi gives him the power to manipulate fire and the power of the fire that stays alive for much as 7 days and 7 nights.