The pilgrimage continues up the Mi’ihen Highroad, now with two new guardians: Auron and Tidus. The party spends the night at Rin’s Travel Agency and Yuna tells Tidus about the goal of the pilgrimage: to go to Zanarkand to obtain the Final Aeon, which can destroy Sin. At the Highway’s north gate the group are not let through because of Operation Mi’ihen, but Seymour appears and gets the Crusaders to let Yuna and the party pass.
The Mushroom Rock is under occupation by the Crusaders, the military arm of Yevon’s clergy. Under the command of Seymour and his fellow Maester, Auron’s former friend Wen Kinoc, the Crusaders join forces with the Al Bhed to defeat Sin. Operation Mi’ihen involves using Sinspawn as bait to draw Sin to Djose, then attack it with a machina said to be as powerful as the aeons. Wakka, being orthodox to Yevon’s teachings and resentful of the Al Bhed, is furious over this plan. Auron stands back, knowing the plan will fail. The operation is a disaster: hundreds of Crusaders are killed and Yuna must again perform the sending. Yevon covers its involvement by blaming the defeat on the Crusaders’ heresy. Auron explains to Tidus the only reason Sin (Jecht) came to the operation was to see his son.
– Final Fantasy Wiki