He hails from the planet Thanagar, homeworld of the hawk-like Thanagarians. Early in the planet's history it was plagued by seven demons and Onimar Synn was the nastiest of them all. He was originally defeated by their great hero Kalmoran, who imprisoned him underground in a cage of Nth Metal (a metal with some very useful properties).
Unfortunately, after a war on Thanagar, Onimar Synn escaped his imprisonment and started to take over the planet. He collected Nth Metal and used its properties to increase his power. He is defeated this time by Hawkman and Hawkgirl using the Claw of Horus together to disrupt his Nth Metal body.
However, it's hard to keep a good demon down and he returns in a later war. This time the heroes who defeat him split his body into seven parts to put in the centre of a different suns to stop him reforming again. Now that is a serious demon.
Sources: Wikipedia | DC Wiki | Hawkman Wiki
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