"Great Betrayer" is the term used for Caleb or the Chosen by either the Cabal or Gideon. It is invoked by calling someone the "betrayer" or by calling someone the "great betrayer". The term refers to Caleb's destruction of Tchernobog. When Tchernobog cast down the Chosen, Caleb fought his way back and killed the god, destroying the Cabal's basis for their religion, and plunging them into chaos and anarchy. At some point, up to the time when Gideon took control, this term came into use.
The term is used in the promotional material found on the Blood II: The Chosen official website in the following quotes:
"They are everywhere, and they consider Caleb and the other Chosen to be The Great Betrayers...--"The Story So Far..." section on Blood II website
"Gideon is unsatisfied. As long as The Great Betrayer, Caleb, lives Gideon feels unworthy of his position."--"Gideon" section on Blood II website.
Here is where the term, as well as the term "betrayer", is used inside Blood II: The Chosen.
"Until a century ago, everything was going according to our... evil plans. We never expected to be betrayed by one of our own."--Gideon's opening Blood II narration
"I shall now witness the fall of the great betrayer!"--Gideon's in-game dialogue, C1L3: Lafayette Museum of Antiquities
"Sorry to break up this lovely reunion, betrayer, but I need to keep you... motivated."--Gideon's in-game dialogue, C3L3: Temple of Poon
"The betrayers will die!"--an in-game recording for the prophets
The term itself is ironic in that Caleb and his fellow Chosen were actually betrayed by Tchernobog first (see Blood).