I will learn to control it, from within. I have all the time in the world. Since you were gracious enough to dispose of Yunalesca... the only means of destroying Sin is forever gone. Now nothing can stop us!
The final battle with Seymour Guado, as Seymour Omnis, is staged inside Sin. Seymour's plan is to gain control of Sin from the inside to wreak havoc on Spira, his pyreflies having been absorbed by Sin.
Seymour Omnis hovers in front of four discs called Mortiphasms. On his turn he will cast four spells in succession on the player party. The spells are determined by the color of the discs closest to Seymour: orange for fire-elemental, purple for ice-elemental, yellow for lightning-elemental and blue for water-elemental.
The spells' strength is also determined by how many discs of the same color are closest to Seymour. For example, if only one orange disc is aligned, Seymour casts a weak Fira spell, whereas four orange discs allow Seymour to cast four powerful Firaga spells.
The Mortiphasms govern Seymour Omnis's elemental attributes:
- Four matching discs allow Seymour to absorb the corresponding element, but render him weak to the opposing element.
- Three matching discs allow Seymour to absorb the corresponding element.
- Two matching discs immunize Seymour to the corresponding element.
- One matching disc halves damage Seymour takes from the corresponding element.
Once Seymour Omnis is attacked six times he will glow red, indicating his change of strategy. He will cast Dispel on the party, and on his next turn cast Ultima that can potentially exceed 4,000 damage to the party. He will rotate the Mortiphasms so that the aligned discs match (but of a different element than before), and continues with his previous strategy. Once Seymour Omnis falls below 20,000 HP, he will do this after only three attacks; counterattacks also count towards this. If his spells are blocked by Nul-spells (and not by immunity or absorption), it is possible to counterattack them. This can lead to multiple counterattacks to a single one of Seymour Omnis's turns; this is the only case in Final Fantasy X where this can happen (the Nul-spells are not a requirement for this to occur; if one simply takes damage it can also happen).
Although Seymour Omnis's defense is initially 180, it is reduced to 100 whenever he casts Dispel. When he uses Ultima, it is increased again, but only to 150.
Seymour Omnis's version of Ultima is different from most Ultima spells in Final Fantasy X. It is somewhat weaker, having a damage constant of 64 instead of 70; the damage is only about 89.4% of what a normal Ultima spell at 35 magic would be. This is somewhat made up for by Seymour Omnis's Ultima dealing special damage rather than magical damage rendering most protective measures useless. It can still be reduced by Shield and/or Focus.
There is a minor glitch: When Omnis has two water elementals, he becomes immune to Fire rather than to Water. This can even lead to a situation where Omnis is both resistant and immune to fire. This persists across all versions of the game.
As the discs face the same way at the beginning of the battle, one of the first actions should be to rotate one by attacking it to prevent a barrage of powerful spells. Physically attacking a Mortiphasm will rotate it counter-clockwise, and using magic will rotate it clockwise. The Mortiphasms, unlike Seymour, are out of range of melee attacks, so only Wakka can attack them physically.
Seymour Omnis has high defense and magic defense, so using Armor Break and Mental Break will make the battle easier. Holy and Doublecasting elemental spells that target Seymour's weakness are also effective, although the latter will only work if the player doesn't attack the Mortiphasms. Casting Hastega or using items or Mixes that grant Haste to the party are useful, however, Seymour may Dispel this, so the player would need to reapply.
The player can let Yuna's aeons do the work: unless Seymour is using Water spells, it is possible to just summon the corresponding elemental aeon who will absorb the spells. Another useful tip is to have Yuna's Overdrive full before the fight and summon Shiva and use her Overdrive without Lulu's Venus Crest (found during a second visit to the Farplane) she will do 9,999 damage if Yuna is trained, or with the Venus Crest on Lulu's Celestial Weapon, she can do anywhere from 0-99,999 damage effectively one-hit KO'ing Seymour.
As Seymour primarily uses elemental magic to attack, it is recommended to equip armor that nullifies or absorbs his spells: the Phantom Bangle for Lulu can be found on the Battle Site, the Phantom Ring for Yuna is found in the Sea of Sorrow, and the Victorious for Rikku is found by inputting the password "VICTORIOUS" on the Fahrenheit. Using Nul-spells is a viable alternative, but this leaves the party open to Seymour's fourth spell.
Another strategy is to use weapons with all four elemental strikes and armor with all four elemental eaters. For example: If Tidus attacks with the Crystal Sword it will do 1.5x damage when the discs rotate for a certain element, and even if Seymour changes for another element, this will happen due to Seymour always being weak to one element when he changes the discs, and with Crystal Shield, most of Seymour's spells will heal Tidus except for Ultima. This strategy with more two more party members will render the player nearly invincible.
By all means, try! You should thank me. Your death means your father's life!
You would oppose me as well? So be it.
But there is no salvation for the damned! Rest in peace, in eternal darkness!
Let darkness take you!
So it is you, after all, who will send me. But even after I am gone, Spira's sorrow will prevail.
The word omnis means "all" in Latin. Seymour Omnis is called "Seymour éthéré" (Ethereal Seymour) in the French version, "Seymour Omega" in the Spanish and Italian versions, and "Seymor Ultima" in the German version.