This is from the walkthrough I'm doing for Chapter 12, which is why it seems to start in a funny way.
Return up to the boat and from Philomene, take about 15 or 16 steps left (that does not include any steps down to avoid obstacles); then go down, there is a slight incline, go down for a chest with the book: Shrine of Yveen. Take 10 steps due right of the chest and go straight down (dodge round a rock and return to the same path). You come to 2 trees close together. A few steps north of the circular cactus on the right of the trees your character will suddenly shout ‘Secret Treasure’ and you have found the weapon Aaghamaniym (ultimate).
Let me know if you find it, then I shall know that the description works. It's been hard finding ways to describe the way to all the chests clearly