This weapon can only be found on the DS remake, on the second floor of Pinnacle Tower, during Chapter 6. 

Preparations

• Finish Chapter 5. You’ll need to finish the game and watch the credits before you can play on this bonus chapter, and find the Cobra Claw. 
• Be at least level 40. This is the largest and most difficult dungeon in the game, so even if you’re making a beeline toward the Cobra Claw, you’ll have to be prepared.
• Equip yourself with end-game gear. Full Zenithian gear on your hero, Liquid Metal Sword and armor on whoever can wield it, etc.

Reaching Pinnacle Tower

You’ll have to cross the first part of this bonus dungeon first. After you reach chapter 6, Zoom to Azimuth town and there’ll be a big hole now where the king is. Get inside to enter the new dungeon.

BF 1

This floor is a linear path through two rooms. Follow it and go down to the next one.

BF 2

Walk north on every bifurcation you see until you reach the last one; turn east now and follow this path to get to the next floor.

BF 3

There are many bridges on this floor. Take the eastern bridge and on the next bifurcation go west. Cross the bridge to take the stairs to the next floor.

BF 4 to BF 7

You’ll find a house that has giant furniture when you get inside. Follow the stairs and you’ll appear in front of another house just like the last one, but this time there is an inn inside. Take this chance to rest.

Inside the inn you’ll see the entrance to BF 6, which is optional. Exit the inn and take the cave north, you’ll find the entrance to BF 7 on the northern end of this cave.

BF 7

This floor can be quite confusing. Follow the first path until it ends, then turn west on the first bifurcation and follow that path, without changing directions at any bifurcation until you see the next set of stairs. 

BF 8

Pretty straightforward. Just turn north on the first bifurcation and enter the tower.

Finding the Cobra Claw

Enemies are actually easier in this second half of the dungeon. 

From here, climb the ramp to the left and follow the path until the northern side of the room, and pick the stairs to the northeast. From here just head south and you’ll find the chest with the Cobra Claw inside.

Congratulations! This weapon can be wielded by both Alena and Maya. It’s one of the best weapons for both of them, not only because it has a pretty good attack, but it also poisons enemies every time you attack. Poisoned enemies lose health at a fixed rate, ignoring their resistance (except for poison immunity). 

You can keep exploring this dungeon if you want. On the next floor there’s a church where you can save and Zoom to whenever you need.