ゴゴゴ is ominous rumbling, so think "rrrrrrrrrrrumbl" or something like that
i've seen "doooooom" and occasionaly "menacing", but i think it's better to come up with an onomatopoeia that suits the mood than to cop out by putting a boring, standard word in its place
Oh, brows on a dragon huh... Should've transformed into a gaping bushy onahole and drop into the pits of hell. Sigh what is with the battle-shonen bullshit in mah fluff... i get the point of the arc but it's dragging on for way too long...