The Great Devoid is a bottomless chasm north of Myrgard. It was created by the Callieach during their final stand against the Trow when they destroyed themselves and their pursuers.
After Balor's defeat in Rhi'anon, Alric sent the rest of the Legion's survivors to The Great Devoid with The Leveler's severed head and they tossed it into the chasm to forever rid him from the world. This would mark the end of the Great War.