This Mysterious Power is the one who captured or imprisoned Blaster Blade, Blaster Dark, and Dragonic Overlord. Before these units were captured, Aqua Force was also captured a long time ago. But Leon Soryu made a contract with Void to release this clan by sacrificing the Royal Paladin, Shadow Paladin, and Kagero clans.
The definition of Void by Takuto Tatsunagi is, "A thing that doesn't have a physical form, existing only as a will and its power is immeasureable....."
In Season 3 a new enemy named "Reverse" appears. Reverse is the avatar of Void once again attempting to destroy Cray and Earth due to both worlds overflowing with life, according to the agent of Void that possessed Takuto Tatsunagi. This is also supported by the creation of Hollow Twin Blades, Binary Star and Binary Star Twin Gunner who were created with data from Blaster Blade, Blaster Dark, and Dragonic Overlord who had been sealed by Void.
It is later revealed that Schwarzschild Dragon of Link Joker was responsible for the sealing of Blaster Blade, Blaster Dark, and Dragonic Overlord, and as a result of this is also responsible for the War of Liberation. In season 4, it is revealed that in order to ensure the eternal continuity of Void, its very essence (the so called seed) is passed on to the one person who is able to defeat Void's current agent. This way the chance to extinguish Void and the manifestation of its will - "Link Joker" - decreases every time the current host is defeated, as Void grows stronger as it passes through stronger and stronger hosts.
Later in episode 194, The Seed of Void is shattered by Blaster Blade, and after the seed was destroyed by his sword with everyones feelings the small parts of the seed flew into the bodies of the people who were there at the time, Blaster Blade's power also allowed the seed to be split even further so that eventually the seed will lose its evil influence.
Later in Cardfight!! Vanguard GZ it is revealed that the Void is controlled by Dragon Deity of Destruction, Gyze.