Either a diminutive of Anastasia, or else from a surname that was derived from Stace, a medieval form of Eustace. As a feminine name, it came into general use during the 1950s, though it had earlier been in use as a rare masculine name.
People with the name Ilyas have a deep inner desire to use their abilities in leadership, and to have personal independence. They would rather focus on large, important issues, and delegate the details.