Macalania Temple
Once you enter Macalania Temple, you will get a cut scene with Shelinda. The scene will differ, depending on if you told Shelinda on the Thunder Plains that Yuna and Seymour were getting married or if you said they weren't. Afterward, a temple acolyte will stagger out of one of the side roomsand say that she found a sphere of Lord Jyscal Guado in Yuna's belongings. Enter the room and you will get a cutscene and view the sphere's contents, which tells of Maester Seymour, Lord Jyscal's son, and Jyscal's murder. Enter the Cloister of Trials (Which at the moment is only a frozen bridge, you will have to complete the trials on the way out.) to view a scene with Auron, Wakka, and Lulu. Cross the bridge into the next room. After an edgy cutscene, you will have to battle Seymour and his Guado Guards.
Maester Seymour
Hp:6000 Mp:???? Ap:~6000ish
Guado Guard x2
Hp:2000 each Ap:????
Strategy:The battle will begin with the 2 Guado casting Protect on themselves and Seymour casting Shell on himself. Change your party to Wakka, Tidus, and Yuna and use their "Talk" trigger commands to double Tidus' Attack and double Wakka and Yuna's Magic Defense. Be sure to kill the 2 Guado Guards first, and fast as they will use Hi-Potions on both Seymour and themselves. If you can, use an Aeons Overdrive (particulairly the un-named Aeon Yuna just obtained) to end this battle quickly. Anyways after Seymour takes a few thousand damage, he summons his Aeon Anima, which you must now fight instead.
Anima
Hp:18000 Mp:Dunno Ap:Lots
Strategy:Ok this boss battle will be a cut above what you've done thus far, but don't get stressed out because it's much easier than you think. He has an instant death attack that is pretty much all he uses called "Pain". This is why I recommend using Aeons (preferably not Valefor). Use one of your Aeons and get them to throw everything they got at Anima and use their overdrives(which should all do about 9999 by now) whenever they are available. Eventually he will fall.
Now Seymour will be back at full health and you must fight him again. The only differences this time are that he has no guards (even if you didn't kill them the first time.) and uses 4 new deadly spells called Multi-Fira, Multi-Blizzara, Multi-Thundara, and Multi-Watera, which involve him casting said spell 2-3 times in a row. After you kill him he will fall to the ground and be carried out by his Guado Guards.
Now you have a chance to name your newly obtained Aeon Shiva. After exiting the room onto the ice bridge, it will decintegrate and you must enter the Cloister of Trials for real this time.
These trials aren't too complicated, but getting the destruction sphere here can be difficult.