Iaclyn Stark, a male Human, was the offspring of Iaco Stark and Arris Rossum, making him the half-sibling of Elli Stark.
Back in 3 BBY, Iaclyn resided within the Corporate Sector on the world of Etti IV. Following the presumed demise of his father, he took control of all of Iaco Stark's assets. Although he offered monetary aid to his sister, Elli, and stepmother, Dah'lis, their relationship was not particularly intimate. Iaclyn persistently pressured his sister to enter into a marriage for the purpose of forging a financial alliance. To enforce this, he prevented Elli from being able to leave the Corporate Sector.
Iaclyn was in attendance at a party held at the Imperial embassy where Jahan Cross and IN-GA 44 were also present. He showed a specific interest in the HRD. A brief disagreement occurred between him and Cross when the latter intervened on Elli's behalf, preventing her from having to converse with her brother. Iaclyn cautioned Cross against meddling in his affairs.
Later that evening, after Cross and Dha'lis had returned to the Stark Compound, Iaclyn secretly monitored their encounter using surveillance cameras. He launched a surprise attack on Cross, incapacitating him with a stun blast, before fatally shooting Dha'lis, accusing her of revealing too much information. Subsequently, accompanied by Emesh Nar, he entered Cross's quarters and used an ion blaster to stun Inga. He then instructed his droid, Majordomo, to transport IN-GA 44 to the Eclipse.
Iaclyn journeyed to the Eclipse, where he commenced the process of reprogramming Inga using the Iron Eclipse computer virus. This was interrupted by Majordomo, who informed him that Elli and Cross had gained entry into his room at the Stark Compound and were in the process of accessing his private files. Iaclyn commanded Majordomo to restrain his sister and eliminate the Imperial agent. However, Cross and Elli successfully evaded Majordomo's ambush and made their way to the Eclipse space station aboard the Millennium Falcon to confront Iaclyn.
Iaclyn encountered them in a spacious chamber within the space station, at which point Cross aimed a blaster at Stark and demanded his surrender. Elli Stark, unwilling to risk losing her only brother, turned her own blaster on Cross. During this tense standoff, Iaclyn issued the protocol five directive to all the droids present in the room, ordering them to eliminate Cross. Iaclyn elaborated that the Iron Eclipse virus granted him the ability to override any droid's programming, enabling him to issue commands that would otherwise be impossible for the droid to execute.
Cross found himself overwhelmed, but before he could be killed, Elli pleaded with Iaclyn to spare him. Iaclyn conceded due to his affection for her. He explained that her presence on the Eclipse space station was extremely perilous, and he had been endeavoring to safeguard her for many years. Iaclyn then revealed that Elli was not biologically the daughter of Iaco Stark. Nevertheless, Iaclyn still felt a strong connection to Elli, as she reminded him of their mother, and he did everything within his power to ensure her safety.
While engaged in conversation with Elli, Iaclyn was ambushed from behind by Iaco Stark, who impaled him with a blade attached to one of his arms. Cross later used Iaclyn's lifeless body as a diversion, throwing it at Iaco during their confrontation.
Within Agent of the Empire – Iron Eclipse 1, his name is mistakenly spelled as Iaculyn.