Darth Krayt, the Dark Lord of the Sith, had ordered the genocide of the Mon Calamari to punish the species for supporting Galactic Alliance Remnant Admiral Gar Stazi, particularly for Gial Gahan's role in the Alliance theft of the Imperious-class Star Destroyer Imperious. In the following weeks after the initial massacre of the Mon Calamari Council, the Mon Calamari were placed in forced-labor camps, where many died of disease and starvation.
Some Mon Calamari managed to escape underneath the planet's oceans. To continue the genocide, the Sith Empire used a number of underwater craft to wipe out all the refugees. But the Empire had numerous encounters with the planet's native resistance units: the Mon Calamari Rangers. The Rangers, in turn, were joined by Imperial Knight Treis Sinde, who acted as a warrior and military advisor for one of the ranger units.
To further the genocide, Darth Wyyrlok suggested to the Sith Scientist Vul Isen that he create a Leviathan that could move underwater. With this advice, Isen created a Sea Leviathan. After demonstrating its power by unleashing it on one of the work camps, Isen and Darth Azard, the Sith Lord in charge of the genocide, set out with the Leviathan to exterminate the Rangers, heading for a spot where an Imperial unit was defeated by the Rangers earlier.
A Ranger patrol ran into the Leviathan, and only one survived to warn the Rangers. While Tanquar, the Ranger unit's leader, wanted to immediately confront it, Master Sinde advised against it. The Sith were setting the Leviathan on a group of refugees to try to draw the Rangers out so they could be killed. After briefly arguing about the situation, Sinde came up with a plan to deal with the Leviathan.
When the Leviathan and the transport carrying the Sith arrived at the Rangers' base, the Rangers, as expected, swarmed out to meet them. But to Azard's surprise, the Rangers attacked the transport rather than the Leviathan. Isen thought this was a foolish tactic, since the rangers' Krakana underwater fighters weren't built to destroy their transport. Azard gave the order to fire on the Rangers.
The Leviathan, meanwhile, attacked the refugees and the Rangers escorting them to safety. At that moment, a large swarm of devilsquid emerged from a nearby crevasse. But instead of attacking the Mon Calamari, the squid swarmed the Leviathan, having been commanded by Sinde to do so. The Imperial Knight also joined the attack on the Leviathan.
When the Sith noticed the Leviathan was in trouble, Azard ordered for the transport to launch their fighters to deal with the Rangers while the transport attacked the squid. But when they lowered the shielding to do so, one of the Rangers rammed her vessel into the hangar, causing an explosion that destroyed the transport and causing the head of it to sink to the bottom of the ocean. Meanwhile, Sinde was rescued by another Ranger while the Leviathan was pulled into the squids' lair inside the crevasse, where it was most likely eaten.
Isen and Azard, having both been inside their transport's head when it was destroyed, survived the explosion. Azard, however, was secured to a chair. Isen took up Azard's lightsaber and used it to free his fellow Sith, much to Azard's surprise.
Sinde later contacted his Emperor, Roan Fel, and told him that their forces should be prepared to deal with Leviathans in case the Sith unleash more of them.
The battle was featured in the 2009 comic Legacy (2006) 33, which was written by John Ostrander with art by Omar Francia. It was the second and final issue of the Fight Another Day story arc of Star Wars: Legacy.