From Final Fantasy Wiki

Operation Mi'ihen
Final Fantasy X
Captain Lucil fighting during the operation
Location:	Mushroom Rock
Result:	Sin Victory Crusaders were excommunicated
Combatants
Al Bhed The Crusaders	Sin
Commanders
Seymour Guado, Wen Kinoc, Captain Lucil	Sin
Strength
The Crusaders 3 Chocobo Knights 1 Black Mage 2 Summoners 1 Samurai 1 Dragoon 1 Ranger 1 Warrior Al Bhed	Sin Sinspawn 1 Sinspawn Gui
Casualties
Heavy, near total	Sinspawn Gui, many smaller Sinspawn

Operation Mi'ihen was a violent attempt by the Al Bhed and the Crusaders to use forbidden Al Bhed machina weapons to destroy Sin. The attempt ultimately backfired when the Sinspawn they gathered to lure Sin fused together into Sinspawn Gui and then proved fatal when Sin appeared and destroyed the machina and decimated most of the Crusader fleet. Among the commanding officers in the battle was Chocobo Knight Lucil, who managed to survive the attack with comrades Elma, Clasko, and a single chocobo.

It is possible for the player to cause the death of either Gatta or Luzzu during the battle. If the player encourages Gatta to go to the front lines, he will be killed by Sin, but if told to stay behind, Luzzu will be dead in his place.

Aftermath

All of the Crusaders were excommunicated from Yevon, who had condemned their actions prior to the failed attempt. Many of the Crusaders retired, while others went to work for temples of Yevon, including the nearby Djose Temple, to seek forgiveness. Many of its other survivors, believed to be dead, were found by LeBlanc and became members of her syndicate.

Likenesses

It is interesting to note that the idea behind Operation Mi'ihen, that the separated parts of Sin wish to be rejoined with the creature itself, is strikingly similar to the idea behind the Jenova Reunion Theory in Final Fantasy VII. Also of note is that the battle is very similar to Sapphire Weapon's attack on Junon.