The Myth III Manual says this about the Great Devoid:
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A great tear in the fabric of reality left behind by the deathblow of the Callieach. It's depths have no end.
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I would take this to mean that Balor's head (still possessed by the leveller) will continue to fall until the end of time. Since the Leveller is part of this equation, I would think that no one else can ever be the Leveller because of his eternal prison in the great devoid.
Here is the Encyclopedia passage on the Great Devoid:
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GREAT DEVOID, THE
A bottomless pit. It was created when the extinct race of the Callieach destroyed themselves, rather than be hunted to extinction by the Trow. At the end of the Great War, Balor's head was thrown into the Devoid, to rid the world of him forever.
Passages From The Game:
"Balor has been killed before," Alric told us after we raised the Myrkridian standard, "and each time it has only made him more powerful. Our best hope is to cut off his head, and hurl it into the Great Devoid. Only in this way will the world be rid of him forever."
-Myth TFL, Level 25, "The Great Devoid"
A bottomless chasm near Myrgard, created by the Callieach during their final days. Rather than be hunted to extinction by the Trow, the Callieach destroyed themselves and many Trow, leaving behind only the Devoid.
-Great Devoid Description, Glossary, Myth TFL Manual
"The Great Devoid is a chasm near Myrgard that is said to have no bottom. It was created when the Callieach destroyed themselves rather than be hunted to extinction by the Trow. Following his defeat at Rhi'anon, Balor's severed head was brought here and cast into the Devoid."
-Great Devoid Description, Glossary, Myth II Manual
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Zandervix
