Kaggath

"'The last gasp of the Jedi Order will be under the heel of my boot!' - Darth Amorosa" The Kaggath was an ancient, but rarely employed, rite of the Sith. One part duel, one part large-scale dejarik-match, the Kaggath pitted Sith against Sith unto humiliation and death.

Rules
One Sith could challenge another Sith for any reason, be it strife, anger, vengeance, greed or pure dislike. The challenger set the arena, whether a planet, a star system or the entire galaxy, and if accepted by the challengee, the ritual began. The two combatants employed the full force of their armies, bases and ships to outwit and outmaneuver their opponent in an attempt to defeat each other's forces.

The battles had to be fought without any kind of outside help, in a direct confrontation of one Sith's power and assets against another's. The breach of any rule of the rite usually resulted in the execution of the offender by other Sith not involved in that particular Kaggath, If one declined a Kaggath the Sith who challenged the other Sith would get his or her rank.

The Kaggath reached a conclusion when one side was defeated or had surrendered, with the triumphant Sith choosing whether to terminate his opponent or leave him alive with the weight of humiliation. The winner also had the option to absorb all the properties, along with the military assets, of the loser combatant or choose to destroy them. Furthermore, the Sith who prevailed in the Kaggath also had the right to destroy his opponent's name and wipe their records from recorded history, theoretically eliminating any chance for the loser to leave some form of legacy to the rest of the galaxy, resulting in a complete deaminated memory of the defeated.

Old Sith Empire
According to Darth Thanaton, Marka Ragnos participated in a Kaggath during his rise. Ludo Kressh also participated in the rite later on. The Dark Lord of the Sith Tulak Hord, upon winning Tulak Hord's Kaggath, ordered that the name of his opponent be erased from history. Darths Qalar and Victun, members of the Dark Council, once fought a Kaggath that destroyed the Imperial Citadel, and were executed by the rest of the Dark Council as punishment. At some point in his life, Darth Tormen defeated Lord Fatalissar in a Kaggath on the planet Ziost as his opponent was naming the arena in which it would take place, before the event began.

During the Cold War, Thanaton challenged the allegedly corrupt Sith Lord Kallig to a Kaggath. During a duel on Corellia, Kallig won the upper hand, causing Thanaton to flee to Korriban to seek the aid of the Dark Council. Kallig followed Thanaton and finished the duel in the Dark Council chamber, leaving Thanaton near-death to be brutally executed by Darth Mortis due to breaking the rules of the ritual in an attempt to ask for help.

New Sith Empire
Though the Kaggath was not an official part of the Sith Empire, the old ritual was used to end the First Order-Republic War when Darth Amorosa challenged Kylo Ren to a Kaggath over Naboo, wanting the war to be over and done with after her wife became grievously injured. The Kaggath pitted the First Order against the Resistance and Sith Fleet in one massive battle to claim the city of Theed.

The remaining two Knights of Ren along with Captain Phasma joined the battle, though they were slain by Talon, Janna and Finn respectively. The First Order also saw a rebellion of it's Stormtroopers led by Finn and Jannah which sabotaged their ground forces and took several ships.

Darth Amorosa herself was involved in the fighting on the ground, commanding her forces from the Theed Palaces square. When Kylo Ren arrived she ordered her ground forces to a retreat and met him in the Throne Room. There they duelled, with Amorosa mortally wounding Ren, ripping the Force Dyad apart, and then killing him. In victory, Amorosa took control of the entire First Order, killed Hux, and ordered a complete and total surrender.