I have been following the walkthrough by YuGiOhFm2002 / YuGiOhAngel, but it's rather vague in regard to it's location. Anyone know where it is exactly? Both it and the Pink Flamingo Weapon's description only had a short description, the latter being nowhere near where the walk was pointing to IMO. :D
The chest in question is behind a breakable wall in the Garden [past]. However, if you've already become the Sandwraith, it cannot be accessed. If not:
Get to the top of the Garden, where you'd previously turned a lever that opened a sluice gate. From there, make your way to the left, shimmy around the wall, and cross the narrow beam. Smash through the weak wall and claim your reward.