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Summon Durneveir looks to be a lot like Summon Dragon, except for two aspects: it's available early (as soon as you can spare three dragon souls), and it's painted in a very different light. Odaviing's doing you a favor when you summon him. When you summon Durneveir, however, you're doing him a favor, because his only chance to taste the open skies is duration of your shout. It may not mean much to some people, but I'm a lot more likely to use Durne because he actually wants to be called upon.