Fraa Jad Assi is an Edharian Millenarian who is evoked at the same time as Erasmas, Arsibalt, Lio and Tulia. In the end of the novel, it is clearly shown that he is an Incanter and may be a part of the Lineage.
Fraa Jad has the capacity to be in multiple narratives at the same time. Most likely he has, along with other Incanters the ability to be aware across multiple narratives simultaneously, thereby altering uncertainty waves such that optimal solutions to difficulties are found, from Erasmas's ultimate chosen perspective.
On page 827 Fraa Jad states to Fraa Erasmas:
"Much pruning has taken place in recent weeks. I am now absent in many versions of the cosmos where you are present."
This quote implies the possibility that Fraa Jad is in some way omniselfaware across the cosmi.
How about this worldtrack? What are the four of them doing in the Narrative that you and I are in? "I'm in several," Fraa Jad said, "a state of affairs that is not easy to sustain."
What is not elaborated upon in the book is in what manner Fraa Jad arrives at the optimal probability state, nor why it requires the death of some and not of others.