Una and Archimago see a knight riding furiously toward them. He is Sansloy, the brother of Sansfoy slain by the real Redcrosse Knight, who now wants his revenge. Archimago is not too keen on fighting, but he cannot run away in the presence of Una, who believes him to be her knight. So he readies himself for the attack, but his shield with his false cross (because he is not really the Redcrosse Knight, the representative of true faith) can’t protect him and if his horse didn’t shy away, he would have been pierced through. (On a side note, those readers who complained that they couldn’t understand Hilary Mantel’s Wolf Hall, because they didn’t know who was meant by “he”, would have a really hard time here, as Spenser refuses to use proper names, as exemplified by the line “He to him lept” (I,3: 317). I understand it’s easier to use personal pronouns, especially writing such a metrically demanding poem, but it does hinder reading and I am not really sure whose horse did shy.)
So Archimago falls down from his horse, badly wounded and Sansloy approaches him to finish him off, in order to complete his revenge, despite Una’s pleas for mercy. However, much to his surpise, when he takes off the knight’s helmet, he sees Archimago, who in fact is his friend. So apparently Archimago didn’t transform himself, just procured for himself an identical suit of armour. The moral is: girls, always check what is under the armour! Sansloy asks Archimago about the meaning of this, but Archimago has fainted. So he leaves him on the ground and instead concentrates on Una, flabbergasted by the appearance of Archimago. He pulls her down from her horse and her lion tries to defend her. However, Sansloy is not a wimp and “lust did now inflame/His corage more” (367 – 8), so he tears his shield back from the lion’s paws and draws his sword. I can’t find in my (admittedly perfunctory) search anything on this subject, but perhaps the fact that Archimago (the Catholic Church) is mistakenly attacked by a Saracen could refer to the then current topic of the struggle between the coalition of the Catholic countries and the Ottoman empire, indicating that Catholics and Muslims are pretty much on the same level from the Protestant viewpoint?