Vacíospren
Voidspren are the spren of the Voidbringers, as they create a bond. A Voidspren will bond with a lifeform, giving it both power and purpose on Odium's side of the conflict on Roshar. Little is known about them, and some is just speculation.
The Desolations
“ In the aftermath of each one, mankind was broken. Great cities in ashes, industry smashed. Each time, knowledge and growth were reduced to an almost prehistoric state. ”
— Jasnah
The Voidspren, once bonded, would fight against the humans in the devastating wars known as the Desolations. These wars often ended civilization for the people, but they usually won. Once a Voidbringer died, it's voidspren would go to Damnation, to be kept away from Roshar. Their only chance to escape was if a Herald also died in the Desolation. A Herald, in an agreement known as the Oathpact, could open the doors of Damnation in order to escape the captivity and the voidspren's torture. The catch was, however, that the voidspren could also leave, and begin a new Desolation. See more on Oathpact
Bonding
In order to take on voidspren, the Parshendi needed to summon the Everstorm. This is because the only way for the Parshendi to bond with any type of spren, or lesser voidspen was during a highstorm. These lesser voidspren grant less powerul forms such as stormform or envoyform. In order for more powerful voidspren to bond, the Everstorm was needed, a storm that gave off voidlight instead of stormlight.
Fused and Regals
Once bonded, the Parshendi took on the form of the Fused, which were Parshendi that had access to the Surgebindings. Whenever the Fused died, the voidspren could take on another host during the next passing of the Everstorm. They don't go back to Damnation probably because the Desolation is still ongoing. Other types of lesser voidspren that bonded turned Parshendi into a Regal. These voidbringers have forms such as stormform or envoyform.
Thunderclasts
Certain types of voidspren could bond with stone, creating giant stone monsters that pulled themselves from the ground. When bonded, these giant stone monsters were nearly unstoppable. This type of Voidbringer is why Knights Radiant needed Shardblades because ordinary weapons couldn't do much against them. Thunderclasts can easily topple stone walls, as seen in the Battle of Thaylen Field.
Bonding Humans
In the Battle of Thaylen Field, Odium makes it known that voidspren can bond to humans. This bond doesn't seem to give any voidbinding abilities and just puts the bonded individuals under Odium's control. This bond can't be very dependable because it needs the influence of the Thrill to make it possible. Once Dalinar Kholin trapped the Thrill, Sadeas's soldiers, who had just been fighting on Odium's side, started to repel the voidspren.
Other Types
There are other types of voidspren that have been seen leading the new Voidbringers to certain places. These look like windspren, except that they are yellow[1] There are also voidspren that are capturing land in Shadesmar, taking cities there. They don't seem to be able to form a bond, as those that do appear as red shadows. These voidspren have some other goal in the Desolations.
Gavilar
Gavilar could have known about the voidspren, as he hinted to Eshonai that he knew how to bring back the Parshendi's gods, involving a method similar to the actual bonding ceremony.
Notes