Was it blood magic? From what I remember of the description in 2nd edition, there were a lot of shamans that died while trying to keep the ritual going, but nothing about blood magic springs to mind. Was this information that the world learned later?
No, it's the sacrifice of life that is the first hint. Under traditional magic (even ritual), you get your "bang", THEN you get your naptime (or grave time, depending on drain). But the Dancers, they were dropping dead during the tempo of the ritual, long before drain. They were willing blood sacrifices to power, fuel and magnify the ritual.
It has been suggested (and not supported) that this is also the reason Daniel disappeared after the NAN was under way (taking the secrets of the rituals with him). At the time, there was a large vocal minority in the NAN that wanted to continue the war, be damned the consequences, but Daniel saw the Rituals for what they were, corrupted and twisted magic and feared the rituals would corrupt and twist the NAN.... (like I said, this is rumored, but not supported)