Just remember that Forgotten Realms has a long and reliable tradition of killing off Demon Lords and Deities and then bringing them back again.
Not just FR. Remember Orcus? He was killed during the Bloodstone wars, I believe. However, he returned, even more eeebil than before as a dark god of undeath named Tenebrous. Now, he's back full swing.
Death in D&D is rarely eternal, not for creatures as powerful as gods and greater demons.