You will never get to come here again, so make sure you get everything. Check the Al Bhed Primers page for the ones you can pick up in Home and make sure you do so. There's a few scenes to watch first.
Finding Yuna
Outside, check Keyakku's corpse for 2 Hi-Potions, and use the Save Sphere. Go up to your party, and you'll be ambushed. This battle is against a Guado Guardian and 3 Bombs. The Bombs have 2200 HP apiece which isn't too difficult to deplete with Lulu and Auron. You can use Lancet on a Bomb to learn Self-Destruct. After the battle follow Rikku and you'll be ambushed again. This time, it's against a Guado Guardian and two slightly more powerful Dual Horns. If you stole some Smoke Bombs from various enemies in Sanubia Desert, then use them here to give yourself an easier time — otherwise, use Dark Attack with Wakka. Most of the enemies you'll encounter here are vulnerable to various Breaks and Slow (use Slowga for efficiency). After the battle follow your party again. You'll reach a fork — go straight on for 6 Al Bhed Potions, right to enter a room where you'll enter a battle with a Guado Guardian and two Chimeras, then get to solve two puzzles for a Skill Sphere and a Special Sphere, and left through the passages to proceed towards Yuna. The two aforementioned puzzles answers are answer 3 (Yes), answer 4 (Brother), answer 2 (Bikanel Island) and answer 1 (Al Bhed). The other one is 5633.
This place is an awesome place to level up, even though it's under attack. Level up your characters so they know at least the following skills and such:
- Auron should know Armour Break and possibly Mental Break.
- Wakka should know Silence Buster.
- Tidus should know Delay Buster or at the very least Slowga.
- Lulu should know Bio.
- Rikku should be close to learning Mug.
- Kimahri is Kimahri, so make sure he's suitably powerful.
Once you approach the next section you'll learn a bit about the history of the Al Bhed and be attacked by some more Chimeras. After that you'll talk with your party members about why the Al Bhed are kidnapping the summoners, and then you can follow them down the stairs. At the bottom, before you go through the door, make sure you look around — to the extreme left of the stairs from your point-of-view there's a slightly obscured chest that contains a Level 4 Key Sphere, and there's a chest to the right that contains a Level 2 Key Sphere. Enter the door for a sequence where Tidus will learn a truth about the Final Summoning and the Final Aeon.
Afterwards, check the lower left corner of this room for a chest containing 10000 Gil, then go through the door and board the Airship. You'll fly off and watch as Home is destroyed. Unfortunately, Yuna's not around. You've gotta find her.