He is both a hero and a villain!
Bruce (unconnected to the other 50 billion other comic characters named that) was in a jungle to view a solar eclipse, but an evil witch doctor attacked him and fell off a cliff. But before doing so, he managed cut Bruce with a black diamond. This cause it so when ever there was an eclipse, the rightmost two-thirds of Bruce's face would be covered by a blue-grey or purple and he would turn into the evil Eclipso. As Eclipso, Bruce had generic superpowers: super strength and near-invulnerability. He could also fire "beams of black light" via his black diamond. In House of Secrets, he Dr. Jekyll and Mr. Hyde-like misadventures. Eclipso could, later, come out during solar and lunar eclipse, or any object blocking a source light. However, a bright flash of light would turn him back into Bruce.
The Darkness Within
Eclispo was retconned as a demon that was trapped in a giant black diamond called the Heart of Darkness. But in the 19th century, the diamond was made into smaller diamonds and Eclispo could possesses any one that got mad and had a piece of one of the diamond. Notably, he didn't need an eclipse and mere pretended to be Bruce's evil half. During the next century, he got all of the diamonds and planned to destroy them (on his moon base), but decided to use them to mind control the Earth's heroes. He explained he picked on Bruce because of his solar research (Eclispo is weakened by light). At the climax of The Darkness Within storyline, Bruce led an army of heroes to stop Eclispo. Eclispo absorbed the possessed heroes, but the fifth Starman destroyed Eclispo's base (freeing them) and the diamonds were still on Earth.
Eclispo decided to take over the country Parador one person at a time. Bruce Gordon, Cave Carson and Mona Bennet (Bruce's fiance) were sent to invested (although CC was left at the border due to breaking his legs). Various heroes tried to stop him. But when it looked like Eclipso won, it turned out the Phantom Stranger had all the diamonds and fused them back into one big diamond, trapping Eclispo again.
The Spectre series revealed Eclispo was an embodiment of God's wrath. But, he got replaced by the titular character. While the Spectres wants to avenge, Ecilpso wants revenge. It was also revealed Eclispo caused the Great Flood from the Bible. The Spectre used the holy power of God to burn the Heart of Darkness and Eclipso's base to ashes. However he appears in Countdown to Mystery, where possess several heroes to get them to fix the Heart of Darkness. Crispus Allen (the Spectre's newest host) tracked down Bruce. Bruce is later possessed by Eclispo, but later gains control over him, but they can't split up.
Eclipso became an ally to shadow-themed heroes. Alex, mad about his sister's death (caused by Eclispo), melted the pieces of the Heart of Darkness (how it got broken again is unknown), but one, and injects them into himself and used magic tattoos, so when he got angry, despite having Eclispo's powers, he was still in control. He joined Black Adam's loose cannon hero team. During Black's rule in Kahndaq, he fell in love with the female Nemesis, but one of the tattoos are broken and Eclipso took over Alex and killed Nemesis. Alex killed himself, so Eclipso wouldn't control him. Now, Eclipso didn't have a limitless supply of his diamonds. Oddly, Alex never was seen using Eclipso's eye blasts. Alex is brought back to life via magic and the Heart of Darkness was fixed.
Lightning Strikes Twice
Despite the lost of many of the diamonds, Eclipso tried to take over Superman and managed to do so. The wizard Shazam battled the Eclipso-corrupt Superman. Captain Marvel managed to do damage to Eclipso (due to Superman's magic weakness) and shoot him with lightning. Shazam sucked Eclipso into his diamond. The diamond was later seen in the Arkham Asylum with the Atom's insane ex-wife, Jean Loring.
Naturally, Jean became Eclipso's new host. She tricked the Spectre to attack magic-using heroes. Nightshade launched her into the orbit of the sun. Alexander Luthor, Jr. got Superboy-Prime to get the diamond and manipulated Eclipso into manipulating Spectre. Jean was trapped in a time loop thanks to Spectre, but is freed by Eclipso and possessed her, only to be launched back into space. After possessing other heroes and Jean was declared dead, Eclipso returned to Bruce.