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The Omega Ruins, also known as Omega Dungeon in the menu at the Monster Arena, is a large dungeon located within a small group of islands to the east of Spira's main continent in Final Fantasy X. Rather maze-like, like Inside Sin, this place must be mapped by the player. Navigating the dark passages, Tidus and his friends will encounter some of the toughest fiends in the game, including the Master Tonberry and Great Malboro, while also coming across chests, which yield some handy (albeit random) treasures.
At the end of the cave, two deadly bosses await the party's arrival.
A traitor to Yevon, Omega, was executed for treason centuries ago, his malicious spirit still haunting these ruins. His hatred has turned him into the fiendish Omega Weapon. His hatred for Yevon was so strong, he also spawned a lesser Weapon: Ultima.
To be unlocked on the airship map, the coordinates 69-75x and 33-38y have to be found using the search function.
The treasures are randomly either a treasure or a Mimic. If the treasure is a Mimic, the nearby chests disappear. If one opens a chest but leaves the chest group, the other chests will disappear even if the Mimic did not appear. The idea is to avoid a Mimic in all four groups to earn 99 Warp Spheres.
The only way to get 99 Warp Spheres is to save the game before entering for the first time due to "RNG" (random number generator) function, which is something else entirely, and not exclusively at the whim of the "random" factors set in-game at Load or by the console. Upon landing at the ruins, the "odds" are set on whether players can succeed at opening every chest, or not (1 chance for success possible in 240 attempts, without guarantee). Saving (anywhere) after visiting Omega Ruins locks in those odds. Once set, they cannot be changed thereafter for that playthrough. The exception is the PC HD remake version, where players can reroll to get the 99 Warp Spheres by using the save in the ruins (unconfirmed). 
Should one pick wrongly along the way and encounter a Mimic, the player should do a soft reset by reloading the file save and try again. The "timing of opening chests" or "the order in which the player open the chests", do not influence the chance to get 99 Warp Spheres.
It is recommended to use the diagram to get the Teleport Sphere by activating the glyphs. For that, the player must move to the west corridor from the entrance, follow the path up to a doormat glyph on the wall, head north to find a chamber with four chests, head northwest into a side corridor with another glyph, activate it, and return to the "doormat" one. The glyph is now working and it raises a bridge out to a chest suspended above the central stage.
The chests contain the following items:
- Lv. 4 Key Sphere
- Defending Bracer
- Lv. 3 Key Sphere x2
- Defending Armlet
- Friend Sphere x2
- Lv. 4 Key Sphere
- Phantom Ring
- Cactuar Wizard
- Teleport Sphere x2
- Warp Sphere x99
These are the normal treasure chests:
- Al Bhed Primer vol. XXVI (near last four treasure chests)
- Teleport Sphere (activate the two glyphs)
- Friend Sphere
- Magic Sphere (return to where Omega Weapon was fought after his defeat)
- Lower part
- Zaurus, ???
- Zaurus, ??? (Ruminant Mimic)
- Zaurus, ??? (Mech Mimic)
- Zaurus, ??? (Roc Mimic)
- Zaurus, ??? (Basilisk Mimic)
- Zaurus x2, Floating Death
- Master Coeurl x2, Floating Death
- Black Element x2, Spirit
- Halma x2, Spirit
- Machea x2
- Puroboros x3
- Defender Z x2
- Demonolith x2
- Great Malboro (always Ambush battle)
- Master Tonberry
- Ultima Weapon (Boss)
- Upper part
- Before defeating Omega Weapon*In each of nine circular platforms (except the first) there will be a fight. If the player leaves the area and returns, all battles in each circular platform will be reset.
- After defeating Omega Weapon
|"Beyond the Darkness"|
|Trouble with the audio sample?|
The theme that plays in the exterior of the Omega Ruins is "Beyond the Darkness". "Gloom" plays while in the Ruins.