In the DC Universe, the White Lantern has the ability to control all forms of the emotional color spectrum, and it just so happens that Green Lantern's arch-enemy, Sinestro, is the perfect candidate for those powers.
The White Lanterns first appeared as part of the Blackest Night saga in order to confront the threats of the Black Lanterns. When Nekron floods the galaxy with black power rings, the dead rise from their graves to challenge all life across the universe. Heroes such as Batman and Superman fall to their influence. In order to combat them, a being of white light called Entity is released from the Earth. The opposite of Nekron, Entity is the embodiment of the power that gave birth to life in the universe. It empowers the White Lanterns in order to defeat the threat Nekron poses.
Related: Even Sinestro's Heroism Proves He Never Deserved to be a Green Lantern
Before they learn of the White Lanterns, a concept that was created by Geoff Johns and Ivan Reis, various leaders of the respective Lantern Corps band together. They know full well it will take the combined might of all their colors in order to defeat the threat. Since they aren’t exactly friends, Sinestro, who is in charge of the color yellow, consistently clashes with Hal Jordan of the Green Lanterns. But Sinestro's complicated backstory inevitably makes him the strongest candidate to become the White Lantern, as well as the first.
Having formerly been a Green Lantern, Sinestro has a connection to that color. But he was also in love with Arin Sur, the sister of Hal Jordan's predecessor Abin Sur. Their relationship allowed Sinestro to feel hope and compassion, and her demise made him feel rage. All of these complicated emotions set Sinestro up as the perfect
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