Forgotten City & Diamond Weapon

Before you travel back to the Forgotten City you need to make sure that you have picked up the Key to the Ancients. The Key can be found in the ocean and is obtainable after you have completed the Underwater Submarine Battle and received the Shinra Sub as a vehicle. The Key to the Ancients is located in the northern section of the ocean underwater in a small cave. Use the map and the screenshots below to help you find it:

Cave leading to the Key to the Ancients
Location of the Key to the Ancients on the World Map

Jump back into the Highwind and fly it over to the Forgotten Capital / Forgotten City. You do not need to travel through Bone Village or the Sleeping Forest this time as you can land the Highwind right next to the Forgotten Capital on the world map (as shown in the screenshot below). Take a left at the fork in the road and follow the pathway around until you reach the Forgotten City.

Cloud at the Forgotten Capital with the Highwind
Inside the Forgotten Capital

Bugenhagen will immediately begin talking once you reach the open area at the end of the pathway. Travel around the pathway to the right side of the screen and proceed offscreen to automatically reappear at the top of the stairs. Walk down and speak to Bugenhagen to continue the story.

Cloud at the fountain in the Forgotten City
The fountain in the Forgotten City

Exit the fountain and then exit the Forgotten City. There will be a short cutscene involving Rufus, Scarlet and Heidegger back in Midgar with the Sister Ray as you exit. Proceed back to the world map to continue the game. Another short cutscene will take place when you try to board the Highwind.

Midgar with the Sister Ray attached
Diamond Weapon emerging from the water

Your next battle is against Diamond Weapon but you cannot actually fight Diamond Weapon while it is out in the ocean. Bumping into it with the Highwind will just cause Cid to scream at you for attempting to break the Highwind and will suggest that you attempt fighting it on the ground.

Diamond Weapon approaching the land

In preparation for the battle make sure that you equip a character with Command Materia Steal Materia and equip accessories that either prevent status effects, such as your Accessory IconRibbons, or that nullify fire-based magic, such as your Accessory IconFire Ring. Equip powerful Magic Materia such as Magic Materia Ultima Materia, Magic Materia Contain Materia and Magic Materia Comet Materia to each of your characters so that they all have access to non-physical attacks.

Boss Battle: Diamond Weapon

Diamond Weapon starts off the battle with immunity to physical attacks. Focus on using high powered spells such as Ultima, Contain and Comet. Diamond Weapon also has a weakness to lightning-based magic making Bolt3 from your Magic Materia Lightning Materia extremely effective.

Make sure that you steal a new weapon for Yuffie called Item IconRising Sun at some point during the battle.

The most powerful attack you will face is called Diamond Flash and there will be a countdown from 3 before Diamond Weapon will use it. The attack will reduce all of your character’s HP down by 7/8ths but cannot actually kill you.

Boss Battle against Diamond Weapon
Diamond Weapon up close

HP: 30,000
Reward: - None

Diamond Weapon will run away before you actually defeat it. There are quite a few cutscenes and FMVs afterwards.

Diamond Weapon cinematic
Sister Ray firing on the Northern Crater

Travel back to the Northern Crater once you are back in control of Cloud in the Highwind. The energy barrier that Sephiroth had created has now been completely removed.

The Northern Crater being fired upon

The next mission is to stop Hojo by traveling back to Midgar.

Note that this is the last point in the game where you can still complete the Wutai side quest and obtain the MP Absorb Materia.