Pretty sure this was one of those straight to video “feature films for families” type movies. a brother and sister found like a jeweled old medal or brooch that transported them to another world. They met this purple dune monster that was helping them find a sleeping giant on the island. For some reason they had to wake the giant up. At some point the dune monster gets sick and has to go back to his ancestral dunes or he’ll die. The giants name starts with an A. I saw it probably late 90s-early 2000s. It was with other movies like Tiny Heroes and Rigoleto.