During the early days of the Galactic War, Darth Thanaton challenged Lord Kallig, the former apprentice of the allegedly corrupt Darth Zash, to a Kaggath. Thanaton's fear of Kallig's growing power, and his disgust for anything associated with Darth Zash, led him to challenge the rising Sith Inquisitor to a contest that would pit their different power bases against one another. There was no opportunity to withdraw and no opportunity to forfeit; the winner would take eternal glory, the loser humiliation and most likely death.
Though Darth Thanaton had an unfair advantage, being a Dark Council member and having a vastly larger powerbase than Kallig, the rising Sith Inquisitor quickly sought out new allies and footholds. The two Sith Lords avoided one another initially, instead pitting their allies against one another. For Kallig this meant working to rebuild the Silencer project, and thus gain the alliance of Moff Valion Pyron. With the Moffs support he was able to gain other military allies, whilst Thanaton focused on the allies he held among the Sphere of Ancient Knowledge. In time, as the two clashed, Thanaton's powerbase was whittled down by Kallig, his allies killed and his strongholds obliterated, his plans laid to waste by the determined Sith.
Towards the end of the Kaggath, Thanaton met with Kallig on Corellia, challenging him directly in front of other Sith and Imperials. Rather than face a strengthened and fully rested Kallig, Thanaton had first Darth Kallous and then two armored guards attempt to kill Kallig, both of which failed. Only then did he intervene himself, falling close to death beneath Kallig's blade. Breaking the rules of the Kaggath, Thanaton fled to Korriban and the Dark Council for assistance.
The Kaggath was brought to a thrilling end when Lord Kallig defeated Darth Thanaton before the Dark Council on Korriban. Thanaton was executed by Darth Mortis as he helplessly crawled to his fellow Councilors. Kallig was rewarded with Thanaton's seat on the Dark Council.
- Star Wars: The Old Republic (First appearance)