Den Siva participated in the Nagai invasion force, which sought galactic domination shortly after the Battle of Endor. A figure of romance and mystery, Den developed an intense and consuming obsession with the Zeltron named Dani. Ultimately, this obsession led him to betray his own people in exchange for ensuring her survival.
Serving as a forward operative for the Nagai, he was assigned to the Dark Lady Lumiya. He accompanied her to Kinooine when she took Luke Skywalker and the Zeltron Dani captive. Intrigued by Dani, Den requested permission from Lumiya to interrogate and examine her. His hope was to gain insights into the Zeltron species and, perhaps, offer her some knowledge in return.
Den's examination of Dani involved cruel and unusual methods. He repeatedly subjected her to the painful laser beams emitted by an analysis grid. As the agonizing examination continued, Den's fascination with Dani grew, even as she withdrew into a quiet and subdued version of herself.
On one occasion, while Dani was recuperating from another examination, Den discovered a beautiful flower growing from a crack in the rock. He was surprised to find beauty in something so delicate and transient. As a Nagai, he valued strength, efficiency, and elegance above all else, not fragility and peace.
When the Iskalonian named Kiro successfully freed Dani, Den engaged him in a fierce duel. During the fight, both combatants were knocked off the cliff and into a deep river below. Den managed to escape, however, and succeeded in freeing Lumiya, who had been captured by Luke Skywalker.
By this time, the main Nagai force had entered the Galaxy and seized control of Kinooine, allowing Den to rejoin his comrades. Throughout the war, his obsession with Dani persisted. He frequently gazed at holograms of the Zeltron, lost in thoughts about her, her courageous reaction to his tortures, and his desire to capture her once more.
When Commander Knife began to formulate plans for invading Zeltros, Den immediately volunteered to spearhead the mission. Shortly thereafter, Den witnessed the murder of Wing Leader Tai, who was killed by Knife for accepting mercy from Princess Leia Organa on Trenwyth. Deeply disturbed by this harsh act of justice, Den and his forces departed for Zeltros.
On Zeltros, Den launched a surprise attack, capturing Leia and Dani. His Nagai and Maccabree troops ambushed a gathering, rounding up most of the Zeltros elite, including visitors Han Solo, Lando Calrissian, and Chewbacca. However, the attack was poorly timed, coinciding with a Tof assault on the planet. The Tof captured Den and threatened to kill Dani unless he agreed to lead his squadron into an ambush. Although his Nagai force was destroyed, Den assisted Han, Leia, Lando, and Dani in defeating the Tof who had coerced him into betraying his own people.
Having alienated himself from his people due to his actions, Den allied himself with the Alliance of Free Planets. He and Dani formed an unlikely bond, feeling a sense of mutual understanding despite Dani's continued dislike for him. Den participated in the attack on Saijo, where Prince Sereno was captured and forced to surrender, thereby ending the Tof threat. The current whereabouts of Den Siva, as well as the fate of his uneasy alliance with Dani, remain unknown.