Serpent.
Really, if Matt Fraction wanted to force in such a major player for Marvel's interpretation of Asgard cosmology and legend, he really should have been on the Thor title for at least a year or two before building up to Fear Itself. It honestly felt like that there was more explanation and the like done after the event was over than before and during the event, which is whack as hell. In the hands of someone like Jurgens or Simonson or even as recent JMS, Cul could have been given the proper back story and buildup he needed.