At the end of The Hero of Ages, Marsh realizes that Ruin secretly planted a hermalurgic spike in Vin in the form of her earring. He is able to seize control of his body to rip the spike free, allowing Vin to absorb the mists. Sanderson gave the reader all the information about the magic system and left clues throughout the trilogy as to the real nature of the earring, but still, I think most readers did not realize what that earring was until that scene.