Also, just found this in the encyclopedia:
Rune Stones
The markers of the 49 Dreams of Wyrd.
Passages From The Game:
These are the original rune stones of Myth, landmark-sized monoliths thought to be fragments of the One Dream, barrows of the Callieach, or both. Their rough surfaces become reflective if an archmage taps them. Each apparently facilitates study and casting of a single dream, but archmages are extremely tightlipped about any additional properties. Their locations are kept secret by those who know them.
-GURPS Myth, Page 85, "Magic Items"
This seems to be relating the Callieach with the One Dream.
This is interesting to me fora couple of reasons-
It seems to imply to me that the Callieach came very close to uniting the dreams. Perhaps they had actually discovered all 49, but refused to share the knowledge.
Also, the callieach were the dark gods creation. This may have something to do with their refusal to reunite the dream. They were warring with the Trow, Nyx's great creation. And they posessed knowledge of Wyrds Dream, his great creation. Perhaps things are more interconnected than we once thought? I feel a theory coming on here :P (but its toooooo late now).
At any rate, I think this at least explains the source of the Callieach's vast power, and what made them such a fearsome opponent.