A1: OK, I don't know which stone you call "the last stone", but I can tell you where all the stones shall be placed. One stone is placed in a ruin near the waterfall before you come to the Small Farm. Only Grulls are there, so you don't need strong weapons to place the stone there. The other stone is in the Wartok cave, the one you shall save the boy from. When you come to the place where the boy is, you must climb some boxes up a secret edge above the last guard. Place the 2nd stone there. The 3rd stone is placed near the ghost boss Zola Dane is talking to in the start movie when you enter Valley of the Fallen. When you are about to meet him, you go to right side of the path to his throne. When you have placed all the stones, you can get back to the Desert of Tears and get the last Blade of the Order. Assassins are guarding the blade, so be sure you got a strong weapon for them.