The cleansing of saidin was a world-altering event that occurred at the climax of Winter's Heart and was felt throughout the entire world by channelers. In 1000 NE, Rand al'Thor made an extraordinary claim: he believed he had discovered how to cleanse the Dark One's 3,000-year-old taint on saidin. While Rand was inside the redstone doorway ter'angreal, one of the questions he asked the Aelfinn was how to cleanse saidin. The answer to the question is never revealed to the reader, but it is known to be a riddle that Herid Fel said stated "sound principles, in both high philosophy and natural philosophy."