The Sea Leviathan was a Sithspawn creature of semi-sentient nature, crafted through alchemical means by Givin Vul Isen, a scientist serving in Darth Krayt's One Sith Order. Inspired by the original creation from millennia ago by exiled Dark Jedi who became the first Sith Lords, this beast dwelled in the aquatic depths of the Outer Rim planet Dac, with its sole purpose being to kill the native Mon Calamari species as per Emperor Krayt's ordered extermination of their entire race. With assistance from the Quarren Sith Lord Darth Azard, Isen drew the Sea Leviathan from Dac's Soheras Trench and unleashed it against the seemingly defenseless Mon Calamari, who were being held at the Imperial Extermination Camp 28.
In the year 137 ABY, its creation year, the Sea Leviathan was attacked and ultimately destroyed by the Mon Calamari Rangers, under the leadership of Imperial Master Treis Sinde. Although it had successfully massacred numerous Mon Calamari, Sinde managed to coax a school of devilsquid using the Force to turn against the Leviathan. Unable to withstand the devilsquid assault, the monster was subdued and pulled into the depths of Devil's Crevasse, where the devilsquid consumed it.
Specifically bred for the deep seas of the Outer Rim planet Dac, the Sea Leviathan was an aquatic creature created by Givin Vul Isen, a scientist within Darth Krayt's One Sith Order. Its size was considerable, dwarfing both the Aquatic Terrain Armored Transport and the Acklay-class mobile fortress utilized by the Empire's military. The Sea Leviathan possessed pectoral fins near its chin and a taloned caudal fin for propulsion through Dac's waters.
Its gaping maw, filled with rows of lengthy teeth, was surrounded by long tendrils. Similar to its terrestrial cousin, it featured two sets of yellow eyes, one above the other, and four nostrils arranged in a similar manner. Unlike the purplish skin of its land-based relative, the Sea Leviathan had a green hide. Cranial horns adorned either side of its face and head, leading to rows of blister pods that extended from its neck down the length of its body. These pods acted as siphons for life-energy and served as its primary attack method. Due to its creator's specialized engineering, the Sea Leviathan only drew energy from the Mon Calamari species, and its blister pods did not affect any other species. Though bred on Dac, it wasn't part of the planet's natural wildlife. The resident Mon Calamari recognized it as foreign and reacted with terror before fleeing.
Despite its bulk, the Sea Leviathan possessed remarkable speed, often outpacing its prey and even keeping up with underwater attack craft. Like its land-based counterparts, it utilized blister pods to siphon life energy from its victims. The beast actively hunted, and when close to its target, the blister pods glowed with a sickly green aura as they drained the target's life. However, due to Vul Isen's modifications, it only activated these pods when hunting Mon Calamari, relying solely on its size, speed, and teeth for defense against other sentient or non-sentient species.
Sea Leviathans were not naturally occurring creatures. Instead, they were a variation of the terrestrial Leviathan, created millennia prior through alchemical practices by the exiled Dark Jedi of the Hundred-Year Darkness. In accordance with Darth Krayt's ordered extermination of the Mon Calamari, Sith scientist Vul Isen engineered a semi-sentient beast that consumed the life energy of its victims and assimilated their sentience. This absorbed life allowed the Sea Leviathan to think more like a Mon Calamari, instinctively locating them in their dwellings and massacring them. The power of the Force was essential to its creation. Vul Isen and Quarren Sith Lord Darth Azard, through their mastery of the dark side, imprinted parts of their consciousness onto the Sea Leviathan's mind to awaken it. Azard worried that as it gained sentience from the Mon Calamari, it would turn on the Sith, but Isen assured him that their mental imprints would prevent this, ensuring it only hunted its intended prey.
In 137 ABY, Dread Lord Darth Krayt decreed the extermination of ten percent of the Mon Calamari species due to their support of the Galactic Federation of Free Alliances, the former dominant galactic government. He assigned the task of overseeing this genocide to Darth Azard, one of his Sith Lords. Azard traveled to the Mon Calamari homeworld of Dac, where Vul Isen was also dispatched to expedite the mission's success. Initially, Azard was content with simply allowing the Mon Calamari in detention camps to die from starvation or disease.
Isen, however, favored more inventive methods of genocide and was eager to implement one of his schemes to please Lord Krayt. After consulting with the Chagrian Sith sorcerer Darth Wyyrlok, Isen created an aquatic version of the land-based Leviathan to unleash upon the Mon Calamari. Once his oceanic Sithspawn was ready, he and Darth Azard used the Force to awaken it from hibernation in the Soheras Trench and then set it upon a group of Mon Calamari being held at Imperial Extermination Camp 28. Impressed by its efficiency, the two Sith released it into Dac's waters as bait for the Mon Calamari Rangers, freedom fighters protecting their people from the Imperial Sith.
Isen and Azard followed the Sea Leviathan in their AT-AT swimmer to Devil's Crevasse, where it ambushed two Mon Calamari Rangers on a reconnaissance mission in Krakana underwater fighters. Caught off guard, they attempted to gather information about the unknown creature before Adriphar created a diversion, allowing Shonmai to escape and report their findings. Shonmai succeeded, but Adriphar's Krakana was disabled, and he was killed as the Sea Leviathan absorbed his life energy. The creature's existence was revealed when Shonmai returned to the Ranger Grotto and alerted her comrades, led by Imperial Knight Treis Sinde. While Isen, Azard, and their Sithspawn patrolled Devil's Crevasse, the Rangers devised a plan to sacrifice refugees to the Leviathan while disabling the Sith's AT-AT swimmer. They encountered the Sea Leviathan near the Crevasse's outskirts, and a battle ensued.
The beast consumed several Mon Calamari refugees while the Rangers, guided by Sinde, disabled the Sith AT-AT. The Rangers then swarmed the Sea Leviathan to save the remaining refugees, but their escape was interrupted by the appearance of devilsquid disturbed by the commotion. Before the devilsquid could attack the refugees, Master Sinde used the Force to command them against the Sea Leviathan. Programmed to attack sentient Mon Calamari, the Leviathan ignored the swarming squid. Sinde ruptured and destroyed several of its blister pods, weakening its effectiveness. Overwhelmed by the combined assault of the devilsquid, the Mon Calamari Rangers, and the Imperial Knight, the Sea Leviathan was dragged into the abyss of Devil's Crevasse, never to be seen again.
The Sea Leviathan debuted in the ninth Star Wars: Legacy story arc, Fight Another Day, penned by John Ostrander and illustrated by Omar Francia. It was a reimagining of the land-based Leviathan created by Kevin J. Anderson for his Jedi Academy: Leviathan comic arc. Ostrander utilized the Sea Leviathan to revisit the Genocide of Mon Calamari, an event first introduced in Legacy (2006) 22, also by Ostrander. As of now, Fight Another Day remains the Sea Leviathan's only appearance.