I remember doing this quest recently, the ritual gets interrupted, so technically…
Also there was never anything said about what kind of souls, like do critters count?
1 million critters wouldn’t take too long to send 
If you open you your Achievement window ingame, there’s a Statistics tab at the bottom, there’s a total kill counter in there if you were interested in just how many things your character had killed.
How close are you to 1 million? 