Ask him what if the Archdemon shows, and he'll say to leave it to him.

They didn't know for sure that the Archdemon wasn't going to be there, and Duncan seemed to be preparing for the possibility.

But between's Cailan's foolishness and refusal to wait for Eamon's reinforcements, and Loghain's refusal to accept aid from Orlais and his subsequent betrayal, everything went to hell real fast. In any case, there were darkspawn gathering in force so of course the Wardens will be there to join Cailan's army. Duncan knew it was a blight, he just wasn't sure if the archdemon was there.

Ostagar was actually an ideal place to make a stand.

If the plan had succeeded, Duncan was probably hoping that he'd have time to talk some sense into the king and get reinforcements from Orlais.

Duncan was pretty clear on the fact that nobody seemed to be taking the 'spawn all that seriously, but he kept his mouth shut to keep from losing what little favor the Wardens had in the country.

I'm guessing that even Cailan didn't think that it was an actual Blight, just an unusually large raid.

Cailan also may have believed that the numbers of the darkspawn are still small after the last Blight, and by killing what he thinks is the whole horde at Ostagar would prevent the Archdemon from doing anything for a long time.

Duncan didn't really seem to believe the Blight would be ended there and Cailan was full of himself trying to be like the heroes of legend.

Why do Duncan, Cailan, and the lot believe that the blight could be stopped at Ostagar? If the Archdemon isn't there, then obviously the blight isn't going to end no matter what happens on the battlefield.

Dragon age origins the landsmeet