Wow. The explanation is simple, logical, and, I thought, intuitive. An archdemon is "unkillable" because the spirit moves to another darkspawn and makes it over into a new body. A grey warden sucks the energy in and dies due to the taint in their blood. In Origins, you can kill the archdemon without sacrificing a grey warden by Morrigan shunting that spirit to merge with the more pliable soul of an unborn child. You literally see the spirit sucked into the elven god instead.
