[Lord_Bob]

It is covered in detail in the manual.

Levels in Blood Magic/Slaves Per Turn:

Blood-0:
Chance of success:10%, Blood Slaves Collected 1 to 6.
Blood-1:
Chance of success:50%, Blood Slaves Collected 1 to 7.
Blood-2:
Chance of success:90%, Blood Slaves Collected 1 to 8.
Blood-3:
Chance of success:100%, Blood Slaves Collected 1 to 9.

The Sanguine Rod will boost someone with at least one level in blood magic by at least one level in blood for the purpose of hunting.