Theory: Void monsters are summoned by humans and can be permamently destroyed by shardblades.

[Nepene]

http://coppermind.net/wiki/Thunderclast

Thunderclasts seem to rip themselves free of stone during fighting at the most recent Desolation.

http://stormlightarchive.wikia.com/wiki/Midnight_Essence

“ReShephir, the Midnight Mother, giving birth to abominations with her essence so dark, so terrible, so consuming. She is here! She watches me die!”

http://brandonsanderson.com/library/120/Warbreaker-Prime-Mythwalker-Chapter-Four

"The guards say he's some kind of Desicrate demon that has taken over a person's body," Skeer noted. "They're executing him tomorrow."

Devin grew cold, watching the creature Ix. The Desicrate? Night Spinners? The stories said they could call down the Demon God's unholy followers and set them upon unsuspecting mortals. The priests of Hess had always taught against them, warning of their dark rituals.

http://brandonsanderson.com/library/137/Warbreaker-Prime-Mythwalker-Chapter-Seventeen

The way Ix's wound bleeds should be familiar to anyone who's read chapter nineteen of The Way of Kings. Brandon also wrote The Aether of Night two books after abandoning Mythwalker, and creatures of that type made an appearance there as well. This is an example of how Brandon keeps trying out an idea until he finds the right book for it to appear in.

We know from Mistborn that magic flows like water through a turbine. It can be used for some purpose and then reused again. As such I have a theory- void monsters are some sort of summoned creature, as the Night Spinners are summoned by the Demon God's unholy followers, likewise the Thunderclast are voidbrought from stone. If you destroy them in time the magic can return to the priests of the void (voidbringers) and so they are impossible to defeat unaided. The priests will just keep summoning them, wearing you down.

Shardblades are unique in that they destroy spirit and investiture as in when they destroy feeling in a limb- we know that how you connect to others is spiritual so I suspect that feelings are spiritual- and so they stop voidbrought monsters from being re summoned by annihilating the magic of Voidbringers permanently.

This works for a while until the priests of the void can repopulate, bringing a new desolation.

Evidence.

“Taking the Dawnshard, known to bind any creature voidish or mortal, he crawled up the steps crafted for Heralds, ten strides tall apiece, toward the grand temple above.”

They bind vordish creatures? Perhaps preventing them from being re summoned.

"I hold the suckling child in my hands, a knife at his throat, and know that all who live wish me to let the blade slip. Spill its blood upon the ground, over my hands, and with it gain us further breath to draw.”

Perhaps they wanted to kill the descendants of the vordbinders to prevent further deaths, and this caused the knight radiant's corruption?

Edited December 5, 2012 by Nepene