Great One was the honorific name of the magician
s of Tsurani
. The Great Ones, who wore black robes and whose organization was known as The Assembly
, were above the Tsurani empire law, but their prime directive
was "to serve the Empire
The magic employed by them was known as the Greater Path, and they were able to do things that the magicians of Lesser Path in Midkemia could only dream about, such as fabrication of magic rifts.
Pug was probably the first "barbarian" (non-Tsurani) to become a Great One, and he also gave the stagnated way of the Assembly a quite of stir.
For more information, see Raymond E. Feist's "Riftwar" series.