I don't remember the exact section but somewhere in the codex it mentions that during the first blight the archdemon was definitely taken down at least one time, which is how they discovered its power to resurrect itself. I assume the original corpse provided a good deal of blood.
I do not envy whatever poor saps had to go through that nightmarish trial-and-error of fighting hordes upon hordes of darkspawn, only to finally get to the archdemon, only to spend forever fighting the hideously powerful dragon, only to find that it resurrected itself by transferring its soul into the nearest darkspawn, for God only knows how many times before finally figuring out how to kill it for good.
I don't know how correlated the two are, I also can't imagine how much trial-and-error they had to go through to discover how to develop an immunity to the taint in order not to waste away or become ghouls while fighting the darkspawn, and how much trial-and-error experimentation with taint-absorbing (and probably Archdemon-slaying) methods before finally discovering the magic concoction of the Joining ritual; and realizing they needed Archdemon blood to make it work.
"JUST STAY DEAD ALREADY!! Maker in the Black City, we've killed you like a dozen times, and sacrificed several hundred of our men to get Bob here close enough to you to see if your soul would transfer into him and not that darkspawn mook several paces behind you, and to see if it would actually kill you since he has a soul while your mooks don't. It's been nearly a century of fighting you Blighted f*ckers! We are tired, we are fed up. Just stay down this time!"
That would be my reaction by the end. 