The Mirror of Erised
In the Harry Potter films, Dumbledore explains to Harry that the Mirror of Erised shows us the "deepest, most desperate desire of our hearts." (The name "Erised" is "desire" spelled backwards.) Young Dumbledore (Jude Law) glances into the mirror himself in the sequel, where he's met with the face of Grindelwald. He also sees a flashback to the two of them as young men, when they decide to slice open their palms and commit to a blood oath to never fight one another.