More good news, my Lord. The Demon of the Black Circle… it seems it's not the only thing we must contend with. The book says that, once summoned, it may call upon minions of its own, described most-endearingly here as a "vast horde spewing untold death, pestilence and destruction".
We could slay the demons using Emerald Sword of Aarn – had it not been shattered to pieces upon the Day of the Eclipse. My lord, bring me as many shards of the Sword as you can find. I make no promises, but it might be possible to form the blade anew.