Final Fantasy X/Secrets and Sidequests/Omega Ruins
Omega Ruins is the top-level secret dungeon of the game, with a lot of the most powerful standard foes the game as to offer. It's divided into two distinct floors, both with a Weapon boss at the end.
 The First Floor
First of all, before you take even one step away from the Save Sphere, you need to ask yourself a question. That question is:
"Do I have a character with a weapon with First Strike on it?"
If you do not, either leave now and buy one or customise it yourself using 1 Return Sphere. There is one reason for doing this. The Great Malboro here will 1) always Ambush you and 2) always use Bad Breath on an Ambush. This is near unsurviveable when used normally but when the first turn is always it, you have absolutely no warning and can end up with a Game Over before you've even realised it. With First Strike, especially on Tidus, you can have your whole party escape before it gets that chance and save you from Game Over; otherwise, switch Tidus in and Flee, or try and Escape. If you absolutely have to capture Great Malboros it is a very good idea to do so either by having a character with Confuseproof and Berserkproof (or Ribbon) on their armours to save themselves from losing control or to go to Sin and capture them there instead. However, this is not to say that the Great Malboro is the only monster you will come to hate in Omega Ruins. There are many more annoying status-inducing enemies that you will encounter. The following is a list of enemies from elsewhere in the game that you will also encounter, and if capturable count towards their respective areas:
- Defender Xs (these show up in pairs)
- Demonoliths (these also show up in pairs)
- Great Malboros (as mentioned above)
A lot of the original enemies down here are immune to sensors or Scan so any data that has been discerned about those fiends are supplied where appropriate.
- Zauruses are lizards with about 8,000 HP. Their defence isn't too shabby which may mean they survive a hit from Tidus; so two hits kills them. They will also inflict Silence and Petrification with their normal attack, so watch out. This is by far the most common encounter on the first floor, especially near the save sphere, in conjunction with... [Can be killed immediately with Death by Lulu -LR]
- Mimics are initially disguised as treasure chests that you encounter from time to time in any normal battle. However, attempt to Steal from them, and it'll say 'Nothing to steal.' followed by exploding into bits of treasure chest and energy vaguely shaped like any number of random fiends. The shape it changes into determines its stats, but it is most likely armoured and immune to Armour Break. However, killing one of these nets you an excellent 50,000 Gil, which can (and most likely, will) be exploited later on in the game with Gillionaire weapons to increase that to 100,000 Gil per mimic killed. Mimics cannot be captured. [You can Mug this creature which will kill it instantly and possibly give you an item -LR]
- Floating Deaths are flying eyeballs, so hit them with Wakka's ball. They look a lot like Ahrimans and have similar traits to their attacks.
- Spirits have about 12,000 HP are similar to Larvae in that they absorb some elements and they can cast those on themselves to heal them and raise their Magic stat. However, this version of them has Poison Mist which is a Bad Breath-esque attack. This is pretty nasty and may be used either as a normal attack or counter-attack; the normal attack deals about 1200 damage and inflicts Poison, whereas the counter-attack version hits for about 700 damage but reliably inflicts Confusion too. Deal with these quickly.
- Black Elements are the toughest variety of Element, hugely resistant to physical attack and they null all elements. They have three spells they'll use; Berserk, Demi and Flare. This is quite a powerful combination, especially since multiple Black Elements can appear at once and begin to Berserk your party faster than it can be cured. Demi isn't really a problem BUT it can weaken characters enough to be killed in one by Flare, if not already (Flare hurts). Non-elemental magic (Flare and Ultima) or special forms of attack (i.e. Attack Reels, Energy Rain, or Anima's Pain) are the only reliable ways of dealing damage to these guys. If you want to capture them, then there is one simple solution; Silence Attack. Elements are weak to Silence which thankfully renders them completely useless as they have no non-magic attacks. Every turn they get will just immediately progress to yours. Then, Armour Break them and hit them once to kill them.
- Halmas are armoured moles. They inflict about 2200 damage with their ramming attack but this also reliably inflicts Slow too. Armour Break them or use Triple Foul to hamper their ability to hit you. These aren't too problematic on their own due to their slow speed but are quite deadly when they show up with other enemies (most often a Spirit)
- Master Tonberries are like the Tonberries of the Cavern of the Stolen Fayth but this time they've got more health (45,000 HP) and they do more damage with their close-range knife attack. Karma still hits for 10x the amount of enemies you've killed. However, a problem arises that the Master Tonberry halves ALL elements, including Holy. [Use Haste on Wakka, then sleep buster continuously with some other character's hits thrown in, will be dead before he gets to you -LR]
- Macheas have about 10,000 HP, are pretty fast, and inflict Poison with every hit, but are otherwise unremarkable; their physical attack only does about 1200 damage. Despite being pretty easy kills, they're quite rare and often drop some very useful stuff (Level 3 Key Spheres and SOS Overdrive weapons).
- Puroboroses are bomb-type enemies with 20,000 HP. Their normal attacks hit a person for about 1000 damage and they have Firaga which takes off about 1700 damage.
- Master Coeurls have 13,000 HP and usually cast Blizzaga and Thundaga (they null both those elements). However, when low on health, they'll cast the likes of Drain, Osmose, Confusion or even use Blaster which as opposed to their Calm Lands equivalent, inflicts instant death as opposed to petrification. However, their low health allows you to dispatch them before they get a chance to.
- Varunas are new, powerful mage enemies. They have 54,000 HP and will start the battle by casting Haste on itself. Your reply should be Hastega or similar since it will counter any attempt to nullify Haste (either with Slow or Dispel) by recasting Haste. It is resilient to magic in general although takes half damage from the four normal elements and 1.5x damage from Holy. They have a very wide repertoire of magic at their disposal, from Reflect to Dispel, all of the -aga elemental spells, Flare and Death.
As there's only one save sphere in the whole dungeon, you may very well have to venture a way in, then equip a No Encounters armour or weapon and make your way back to save and heal. You'll want to do this regularly as even preventative measures against a Great Malboro ambush might not work against other enemies which appear harmless enough but manage to pull off a chain of good attacks and wind up killing you.
The teleporter down to the bottom floor of Omega Ruins is on the far side of the dungeon from the Save Sphere, and there are numerous things in the dungeon itself for you to get. You may find groups of chests in areas lit by purple light - opening these in the correct order will net you useful items like Level 4 Key Spheres. There is a set sequence of goodies that you may get from the chests, however there is a good chance that some of the chests are traps. You have no way of knowing which chest is a trap. Open one, and you get coaxed into a battle with a Mimic and the rest of the chests in that area will disappear.(NOTE there is no permanent order for these trapped chests). Also opposite the Save Sphere is an Al Bhed code to decipher.
So, to reach the teleporter, head left at the crossroads, then right when you reach the purple room, right at the fork up ahead, left around the corner (ignoring the Glyph here for now), right in the large cavern and then finally use the lift to the right to go up to where the teleporter is. However, you will have to deal with Ultima Weapon before you can go down.
 Boss: Ultima Weapon
This isn't really a boss if your party is moderately powerful, since he only has 70,000 HP. His attacks are basically weaker versions of some of Omega Weapon's attacks, which include a physical attack that does about 3400 to a person and an assortment of weak black magic. Interestingly, you can steal 10 Doors to Tommorow off it for each successful steal, and Bribing it with 1,400,000 Gil will net you anywhere between 60 and 99 Pendulums which can be used to customise Master Thief. Note that players with the PAL version of the game will have to stump up closer to 2 million gil to get their Pendulums.
 The Second Floor
This floor is a chain of difficult random encounters between you and the entrance to Omega's lair. You have two ways to go once you're down here, you can fight two encounters to the left for a Friend Sphere in a chest, or about eight encounters to the right to reach Omega. The encounters you will get are:
- A Great Malboro. This is just like the encounters in the floor above in that it will always ambush you and always use Bad Breath on an ambush.
- Two Demonoliths. Holy works a treat, but make sure you've got some protection against Petrify.
- A Master Tonberry. See the monster list above for details. His counterattack is Karma and it hits a person for 10x the amount of monsters that character has defeated in non-elemental damage.
- A Varuna. Same as above.
- An Adamantoise. Same as above.
- Two Wraiths. The only thing to watch out for here is if you're slow then you will be taking a lot of punishment from multiple Doom and Death spells.
Once you've fought your way there, heal up, and head into the purple haze to confront Omega!
 Boss: Omega Weapon
The version of the game you are playing will determine if this battle is going to take a similar amount of time to Ultima Weapon, or much MUCH longer. US and Japanese version players will only have to contend with 99,999 HP, whereas International version players will be up against a whopping 999,999 HP!
Omega's attacks are quite varied, although they are largely magical in nature so Shell/Mighty Guard works wonders. He has a physical attack which can hit for about 4000 to one person, Confuse which does what it says on the tin and inflicts Confusion on one character, Shimmering Rain which hits the whole party for about 2500 damage, and Core Energy hits one character for up to 6000 damage. This is based on an average Magic Defence stat of about 100.
However, if attempting Omega at low levels you will be in trouble if ever it decides to use its two trademark attacks. Ultima, the most powerful black magic spell, is at its disposal, and this will not only penetrate Reflect but deal up to 9999 damage to everyone! A high Magic Defence stat is thus crucial to success here, alternatively letting Aeons rack the damage up for you whilst having stats able to let them survive its more powerful moves is also a good idea. The other attack to watch out for is Nova, which is the same in that it will deal up to 9999 damage to everyone. However, the upside with this is that Kimahri can use Lancet on Omega Weapon to learn it, thus completing Kimahri's list of available Ronso Rages.
As for beating this thing, first of all, you should note that Omega absorbs all elements except holy. This means that Overdrives from Ifrit, Ixion or Shiva will actually heal Omega rather than hurt it, and if Yuna is even anywhere near as powerful as she should be for this battle then this can restore up to 30,000 damage! If you have to use these Aeons, stick to using their physical attacks (or Flare/Ultima if you've taught it to them) and use them to block Omega's attacks for you. Resist the temptation to use their Overdrives at all costs. On the other hand, coming into the battle with all your characters and Valefor, Bahamut, Anima and the Magus Sisters with full Overdrives allows for high amounts of damage to be inflicted in a short amount of time. It's probably best to give Anima a double Overdrive if you plan on using Grand Summon as Oblivion will likely do more damage than Delta Attack from the Magus Sisters. However, the Magus Sisters are obviously more capable of handling themselves against Omega since there are three of them. Their only drawback is that Mindy will have an obsession with casting -ara and -aga spells at Omega if one of the Sisters has Reflect, which of course heals Omega. Fortunately, Sandy's Razzia, Mindy's Passado and Cindy's physical attack/Camisade should ideally rack up more damage than elemental magic will heal.
A good start to the battle will be to have ideally everyone with their Celestial Weapon fully powered up and with an Overdrive, so your starting set up should be Tidus, Lulu and Auron. Have Tidus cast Hastega, and Lulu should use Flare/Ultima Fury on her first turn, and Auron should use Tornado (if he has it, otherwise Banishing Blade). Once they're both done, switch them out for Rikku and Kimahri. If Kimahri has an Overdrive he should use it (either Mighty Guard or Jump/Seed Cannon) before using Lancet to learn Nova. Before switching him out for Yuna or Wakka, have him use Nova on Omega Weapon. Rikku should use the best Mix she can (either for an attack item like Sunburst or Supernova, or benefitial like Hyper Mighty G) and then switch out for either Yuna or Wakka. Tidus should either attack or recast Haste/Hastega as the Overdrives will push characters far down the turn list. The party that will then sort you for the rest of the battle will be Tidus, Wakka and Yuna. Tidus should recast Hastega then use Blitz Ace (or Slice & Dice if you don't have it). Wakka should use Attack Reels. After that, Yuna should start her long chain of summoning whilst in between Tidus and Wakka attack/heal. Once you run out of Aeons, if Omega is not dead have Yuna start casting Auto-Life (if she has it), Shell and Regen and as many positive status effect spells as possible before Omega can use Ultima or Nova on your party. Of course, if your party has recently been romping through the Monster Arena this battle should not be a problem, as 11 hits at 99,999 (or a perfect Attack Reels at around that) will do him in nicely.
Come back after you were able to defeat him, and obtain a Magic Sphere from the treasure chest left behind. You should not have to run into any of the automatic fights with the monsters listed above in each ring while descending. Regular fights still occur unless you have a "No Encounter" item equipped to one of your characters.