The name of this episode is Mirage. A simple single word title can often open itself up to much interpretation. There is no obvious illusion in the episode. Shiro is not tricked into going to the Ryuudou Temple by a mirage. I think the titular mirage is the illusion of control that Shiro has.
Despite how much he thinks he is in control of his life it seems that more and more he is just being moved like a pawn at this point in the story. Rin is distinctly in control of their alliance. While that arrangement is clearly in Shiro’s best interest it is theoretically an equal partnership but the thing is Rin is in the drive’s seat. We see Shinji play Shiro like a fiddle as he knows all the right strings to pluck on his former friend. And in the end Caster draws Shiro into her little trap all too easily. Shiro might think that he is a knight on the chess board of the Holy Grail War but it is clear that he is merely a valuable pawn. He won’t gain any measurable power or control unless he can survive long enough to get a promotion. Read the rest of this entry ?