From the title, I thought this thread might have been about Alexander the Great.
Except that Alexander the Great was never conquered by "sands."
But the Siwa reference is absolutely connected to Alexander's exploits. The oasis at Siwa was home to the world renowned Oracle of the god Amun (Ammon) described in Herodotus' Histories.
At the beginning of his campaigns Alexander took a side trip there and legend (Herodotus) says was saved by a rare thunderstorm.