Kallig arrived as Thanaton was making a speech to high ranking Imperial and Sith observers, about his adversaries corruption both by association with the presumed dead Darth Zash, and her murder. As Kallig prepared to face Thanaton, the Dark Lord ordered a subordinate Sith and then his Guardians to instead kill his foe.
After Kallig dispatched Thanaton's allies, the two Sith finally fought directly.
After a drawn out duel, Thanaton became frustrated with his inability to defeat the young and inexperienced Sith Lord. Eventually Kallig disarmed Thanaton, who responded by fleeing the field, much to Kallig's surprise and dismay.
It was revealed that Thanaton had planned to flee to the Dark Council should anything go wrong. It was there, before the Dark Council, that the Kallig and Thanaton faced off for the third, and final time.
- Star Wars: The Old Republic (First appearance)