Trogg was some kind of creature trapped in darkness. He was accidentally released when the trolls broke a portal into the darkness of Trogg’s world. Since an unlikely alliance between Ulik and Thor could not defeat him, they resealed the portal and confined the creature back in its own dark world.
The creature was obviously very strong, because the trolls couldn’t construct a door strong enough to hold it. Nor could Ulik and Thor together defeat it. Trogg can produce a blast of flame from its mouth strong enough to send Thor flying.