The Light of Destruction

The Light of Destruction (破滅の光, Hametsu no Hikari), which can also be translated as The Light of Ruin, is a malevolent celestial entity that possessed Sartorius when he first came into contact with the "Ultimate-D card" and the main antagonist of the second season of Yu-Gi-Oh! GX. Highly virulent and dangerous, The Light of Destruction wreaks unholy calamity on the world in the second season and its influence triggers the conflicts of the third season.

Personality
The Light of Destruction's ultimate ambition in the Japanese version has been described as destroying all life in the universe (its been referred as the "light bent on destroying the universe," or something similar multiple times while wishes to dominate all life in it the English dub, going as far as saying that it wants life to "worship" it), but the Light is arrogant in its beliefs that its actions are justified. In the dub it boasts that humans would go on a path to self-destruction despite the fact that it displays no regard for human life. In the original the light boasts that destruction is the destiny of the universe and actually claims that the universe had been destroyed many times before (whether it was lying or not is ambiguous). Those possessed by the Light often exhibit a glow of light around their bodies.

History
For billions of years a white hole released a mysterious energy called the Light of Destruction. When this energy reached Earth it caused several calamities such as dictatorships and wars. Ten years before the events of the second season, the Light of Destruction was released, infecting a card created by Aster's father, "Destiny HERO - Plasma". When The D acquires the card by murdering Aster's father, it possesses him, causing him to develop an uncaring and sadistic alter ego.

Afraid of what is happening to him, The D consults with a fortuneteller named Sartorius. The Light takes Sartorius as his main host instead, but a small portion remains within The D, which helps him to ascend the Pro League and become world champion.

Society of Light
After possessing Sartorius, it sets the stage for the events of the second season by amassing a cult of brainwashed students and acquiring a satellite capable of extinguishing all life on Earth (in the dub, the satellite would cause a mass-brainwashing of all Earth's populace).

It later partially infects Prince Ojin, who had been brainwashed to serve Sartorius in the Society Of Light. It is ultimately sealed away, freeing Sartorius from its influence due to the efforts of Jaden and Aster. However, only that part of it was destroyed and the Light still exists.

Dimension World
Although it didn't play the same role in the third season, it is eventually revealed that Yubel came into contact in with The Light of Destruction while it was in space, which distorted its sanity although it did not control her like it was able to control Sartorius. This is probably because Yubel was a spirit of darkness so being exposed to that malignant entity probably had unnatural results. It's also revealed that Jaden had always been the one destined to defeat the Light with Yubel being specifically created to protect Jaden's past self The Supreme King until he was old enough to utilize his innate power to do so.