Another odd part about the answer to their Calling is to raise a demon army. If using demons to fight for you is such a good idea, why haven't they tried doing that in previous Blights? Surely having an army at your disposal that doesn't get tired and can't be affected by the taint would be ideal, particularly as you wouldn't have to bother with all that sucking up to local nobility in order to get them to supply forces to you. Yet that would seem to be the one thing they hadn't tried doing. So now, when there isn't actually a Blight happening and so really no need to panic, suddenly they think they have no alternative but to listen to the advice of some random Tevinter mage.
It was a stupid plan plain and simple , yes you can do stupid things when desperate but come on.
There is TWO archdemons left , no blight on the horizon , the wardens hear the calling but they don't know what the hell is going on.
They decide they are all going to die , before checking out the wardens in the north are also hearing it.They weren't ...
But anyway they decide with no proof that all wardens everywhere are going to die in a couple of weeks/months.
There is no Blight but they also decide they are the last wardens and must take down the archdemons no matter the cost ASAP.
Basically they heard the Calling and jump to conclusion the apocalypse was just around the corner.
It's kinda bad but I can at least understand the "Something very wrong is going on" gutfeeling they had.
Thing is on top of that , they trust a random Tevinter mage who happens to be there with some kind of plan.
Now if it were me at this point I'd be paranoid , the calling is weird and the mage is weird.
Then the marvellous plan...and this is where I don't understand how we're supposed to keep a straight face.
So the plan:
The warden warriors will kill themselves (brillant!) the mages will then use their blood to summon a demon army.Remember the wardens are freaking out because they think they are about to die ...but dying for some blood magic spell?Sure.
Now let's look at our demon army.You need to keep those under control at any cost , so it means the mages have to keep watch and be in control.
Now those mages and demons will need to go to the Deep Roads for a very long period of time , it takes hundred of years for darkspawn to dig up an Archdemon (and there's two left , not at the same place!)So they need to survive the Deep Road , face down a massive horde of darkspawn , not loose control of the demons , dig a lot , then kill an untained Old God (nobody has ever done that before so who knows what would happen) oh and also survive that Calling for months and then they need to do it a second time ...
For this very very stupid plan to have an actual chance to work they'd also need to get in touch with wardens elsewhere so those can prepare for Archdemon number 2...but oh no if they did that they would have figured out not all the wardens were dying and they didn't need to summon demon.
So yeah Derp derp warden derp.