Illidan should not have the skull of Gul'Dan in the first place, at the end of the second war when Khadgar closed the dark portal from inside (He closed it while he was still in Outlands) he had to use the skull of gul'dan among other magical relics in order to cast the spell and close the portal. The portal closed and khadgar dropped the skull onto the ground. Years later Illidan MAGICALLY found the skull in felwood, when it should have be somewhere in Outlands. There's no explaination on how the skull could have traveled from Outlands to Azeroth, and then from the Eastern Kingdoms to KALIMDOR, all while the portal was closed.