Warning spoiler in this post from halo books and other sources
the biggest theory was that a group called The Servants Of Abiding Truth (a sangheili group of monks that see the forerunners as gods) were going to form the storm, overtake the Vadum keeps, and declare war on humanity who also in accidental way kills hois wife. However once The Thursday War came out, clrity came. there is a Sangheili by the name of Jul 'Mdama who gets captured bythe humans. to make a story with little spoilers ill make it short. he is trapped by humans on a world with forerunner technolegy. through this technoledgy he learns of the Didact and how he hates humans. thru the technoledgy on the planet he escapes to a sangeili planet where he finds out where the location is of requiem is and with the help of the elites on this planet forms the storm and goes to find the didact to go wreck havoc on humanity getting his revenge.