Eclipso is a character in the DC Comics Universe.
Origin
Eclipso debuted in House of Secrets #61 (August, 1963). Eclipso started out as a scientist, Bruce Gordon, who visited a remote Pacific island called Diablo to observe a solar eclipse, and got into a tussle with a local witch doctor. The native was killed in the fracas, but Gordon escaped with only a minor cut from a black diamond. However, during the eclipse, Gordon was transformed into a super-powerful being who embodied the evil side of his personality — Mr. Hyde to Gordon's Dr. Jekyll; or, in comic book terms, The Hulk to his Bruce Banner. The black diamond became a means to focus destructive energy blasts from his eyes. Only his colleague, Simon Bennett, and Bennett's daughter Mona, knew his secret.
The transformation happened again during the next eclipse, and again and again thereafter. Lunar eclipses triggered it just like solar ones. In fact, even an "artificial eclipse" — someone passing a hand in front of a light bulb — would do it. Fortunately, Eclipso wasn't too difficult to banish. A sudden flash of bright light would turn him back into Bruce Gordon.
House of Secrets folded with its 80th issue (October, 1966), and with it, both of the series it had been carrying (the other being Prince Ra-Man , a minor and decidedly oddball superhero). The title was revived a few years later, but the running characters remained gone. Eclipso was next seen seven years later, fighting The Justice League of America. He went on to occasional bouts with the likes of The Phantom Stranger and The Metal Men, and briefly, during the early 1980s, became a supporting character in Green Lantern. But he didn't become a major powerhouse of a villain because, as later writers said, it was hard to take a character seriously when he could be defeated with a flashbulb.
Because of this, he was revamped in the 1990s. In Eclipso: The Darkness Within, a 1992 crossover event that involved Superman, Starman, The Creeper and many other DC characters, they powered him up and divorced him from Bruce Gordon. According to this retcon, Eclipso is an immortal god of vengeance and, from his castle on the dark side of the Moon, can manifest himself through any of a thousand black diamonds, turning anyone who thirsts for revenge into a version of himself. The name "Eclipso" no longer referred to his ability to manifest himself during an eclipse, but to his propensity for "eclipsing" a personality and submerging it within his own.
Post-Crisis
Eclipso had his own comic from 1992-94 and, now quite powerful enough to take seriously (he has even succeeded in permanently killing a few of minor superheroes), is a familiar sight to readers of the entire DC line.
Eclipso, an evil god of revenge imprisoned from barbarians primitive race of reptilians in a Black Diamond. Eclipso was the original God of Vengeance who turned evil after being replaced by Spectre. He appeared in Justice League Animated and House of Secrets Vol. 1 #61 (July-August 1963). Eclipso was also prevelant in the Eclipso short-lived series for completely killing off a group of superheroes known as the Shadowfighters .
Eclipso has also appeared in a Justice League Animated episode called 'Eclipsed'. He also participated in the "Black Reign" storyline, teaming up with Mordru and Obsidian.
In the upcoming Day of Vengeance miniseries, which will tie into the Infinite Crisis, Jean Loring , ex-wife of Atom Ray Palmer, discovers a black diamond in her prison cell and becomes the new Eclipso.