Despite seemingly being one of the most powerful characters in DC's library, Synnar (assumedly pronounced 'sinner'), has only mustered a couple of appearances in the publisher's comics. He's not even appeared in the current New 52/Rebirth canon, which may make sense, given his powers slightly... kind of... break everything.
You see, Synnar was originally the creator of the multiverse. He was God's architect, but once he created everything, he conspired to kill God and was punished for his troubles. He eventually reconstituted himself as Synnar, and first made his presence known to DC's heroes in the pages of Hardcore Station, where he revealed to Hawkman the in-world explanation as to why his history is so fangled.
Sadly, the fact we've seen so little of the character means that the true extent of his powers are still unknown. We know that he's a god, and that he literally created the known universe, but it would appear as though those abilities were stripped away from him after being taken down by the almighty.