According to the legends, they cast mankind out from the Tranquiline Halls to Roshar and, from there, they tried to force mankind out of Roshar to Damnation. Additionally, according to the legends, they were defeated during Aharietiam.
According to legend, the Shardblades that were first carried by the Knights Radiant uncounted ages ago, were gifts of their god, granted to allow them to fight horrors of rock and flame, dozens of feet tall, foes whose eyes burned with hatred. The Voidbringers. When one's foe had skin as hard as stone itself, steel was useless. Something supernatural was required. This legend most likely refers to Thunderclasts, which are possibly a type of Voidbringer. They might be Parshendi bonded with Voidspren, such as stormspren. There are likely ten types of Voidbringers, just as there are ten orders of Knights Radiant.
In Words of Radiance, it is incorrectly revealed that the Voidbringers are the Parshendi when they assume stormform, characterized by their red eyes. The parshmen can also transform into said Voidbringers, though they lack the ability (or initiative) to transform on their own. The Everstorm carries spren that will supposedly turn parshmen into Voidbringers on contact.
According to Kalami, the Voidbringers came to annihilate. She tells Dalinar that their goal was to wipe humankind from Roshar. She elaborates by saying that they were specters, formless ... that some say they are spirits of the dead, others spren from Damnation.
Jasnah was most interested in the Thaylen parshmen who had stolen ships that had survived the storm. Their exodus - combined with Kaladin's interactions with the parshmen in Alethkar - was painting a new picture of what and who the Voidbringers were.
The document, Eila Stele, reveals that the Dawnsingers were Parshendi and the native inhabitants of Roshar. Apparently, humankind came to Roshar seeking help after accidentally destroying their own world (i.e., Ashyn) with Surgebinding. Commanded by their gods and motivated by their pity, the Dawnsingers helped humankind, only to be betrayed. They called humankind Voidbringers, for they had brought a void that sucked in all emotion.
|“||"Voidbringers have no pattern. Besides, I have read of them in your lore. They speak of spindly arms like bone, and horrific faces. I should think, if you wish to find one, the mirror might be a location where you can begin your search."||”|
|“||"They were creatures of terrible destructive power, forged in Damnation, created from hate."||”|
|“||"Everything has its opposite, Shallan. The Almighty is a force of good. To balance his goodness, the Cosmere needed the Voidbringers as his opposite."||”|
–Kabsal to Shallan