You're not playing as the Warden, or the "Champion of Ferelden," and he has not been in any of the trailers. You're playing as Hawke, "Champion of
Kirkwall," who's been the focus of all the advertisemant. And I don't know what trailer you're talking about, but I'm assuming http://www.youtube.com/watch?v=jlACgYHtWCI&feature=related. Let's clear a few misunderstandings: those horned guys are
not Darkspawn, they're Qunari. And the spell Hawke uses to finish the Qunari off is nothing special, just some finisher move for mages which is available in the game.
Also, the
God Baby ending is not being made canon (unfortunately). There are three default scenarios if an Origins save is not imported, and only one of them leaves the possibility of that being the ending of choice.