|Final Fantasy X Boss|
|シーモア:最終異体 (Shīmoa: Saishū Itai)|
|Sensor||Energized by the Mortiphasms.|
|Location||Inside Sin - Garden of Pain|
|Common Steal||Shining Gem|
|Rare Steal||Supreme Gem|
|Common Drop||Lv. 3 Key Sphere|
|Rare Drop||Lv. 3 Key Sphere|
|Weapon Drop||Piercing, Magic +20%|
|Armor Drop||SOS Shell, SOS Protect, SOS Haste|
|Abilities||Fire, Thunder, Blizzard, Water, Fira, Thundara, Blizzara, Watera, Firaga, Thundaga, Blizzaga, Waterga, Dispel, Ultima, Banish|
|Resistant to||Zanmato (Lv. 4)|
|Immune to||Silence, Sleep, Darkness, Poison, Petrification, Slow, Zombie, Power Break, Magic Break, Threaten, Death, Provoke, Doom, Distil, Gravity, Delay, Eject|
|Other Information||Seymour Omnis's elemental affinities depend on the positioning of the Mortiphasms.|
- "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, the battle against Seymour Omnis, is staged Inside Sin. Seymour's plan is to gain control of Sin from the inside to wreak havoc on Spira, but how he managed to get inside it is never addressed in the game.
Seymour Omnis hovers in the air in front of four discs called Mortiphasms. As each disc rotates, the colored sections determine which spells he will cast at the party each turn. If each color is the same, the spells will have increased power. Seymour's magical weakness is determined by whichever side is furthest away from him when he is attacking. After each attack, the discs will rotate so a new spell is used.
As the discs are facing the same way at the beginning of the battle, one of the first actions should be to forcibly rotate one by attacking it; this will prevent a barrage of powerful spells. Physically attacking a Mortiphasm will rotate it to the left, and using magic will rotate it to the right. The Mortiphasms, unlike Seymour, are out of reach, so only Wakka can attack them physically.
After six rounds, Seymour will begin casting multi-Dispel followed by Ultima that can potentially do over 4,000 damage to all characters. By this point, Lulu should know Doublecast and Flare, and Yuna will have obtained Holy; both can be used to great effect.
One useful method is to start out with Lulu, Yuna, and Rikku in battle. Equip the Phantom Bangle (found at the Battle Site) on Lulu, equip the Phantom Ring (found in the Sea of Sorrow) on Yuna, and equip the Victorious (found by entering the passcode "VICTORIOUS" in the airship) on Rikku. That way, Seymour's elemental spells will mostly have no effect or be absorbed by the three.
Another method is to use Armor Break on Seymour and then summon Anima in Overdrive mode and simply have her perform Oblivion, as this will deplete well over 50% of his HP or may or even kill him in a single hit.
A final option involves casting Reflect on all party members and continually attacking the discs physically so they never line up. In this way, Seymour never receives full immunity, so every time he casts a spell it will bounce back and damage him instead.
- "By all means, try! You should thank me. Your death means your father's life!"
- —Seymour to Tidus right before the fight begins.
- "You would oppose me as well? So be it."
- —Seymour to Anima if she is summoned during the fight.
- "But there is no salvation for the damned! Rest in peace, in eternal darkness!"
- —Seymour to the group before casting Dispel the first time.
- "Let darkness take you!"
- —Seymour to the party before casting Ultima.
- "So it is you, after all, who will send me. But even after I am gone, Spira's sorrow will prevail."
- —Seymour to Yuna after his final defeat.