Sedín, a member of the Caeliar, was a companion of Inyx for several eons. They made a daily ritual of watching the sunsets while living in Axion. Sedín disapproved of Inyx swaying the Quorum to take custody of the Columbia in 2168, and felt that displacing the ship and its crew would be a better means of protecting the Caeliar. Inyx felt that Sedín was merely overly anxious about any new species they encountered. (ST - Destiny novel: Gods of Night)
Even after the Columbia were able to approach and send an away team to the planet's surface, she thought that Inyx had defiled their home. Inyx thought of her many times after the loss of Erigol, wondering if he had made the proper choices. However, he drew inspiration from Captain Erika Hernandez's willingness to help even when distraught. (ST - Destiny novel: Mere Mortals)
At the time of the Great Work destabilization, Sedín was at Mantilis and was able to escape along with the Earth Starfleet personnel and eleven other Caeliar, surviving the crash landing onto Arehaz. After only one day on their new home, the Caeliar began to feel the ebb of their vitality away from the sustaining power of Erigol. The twelve discussed alternative sources of energy; Sedín dismissed the idea of geothermal power as the bedrock was deep and they were too far from volcanic activity, suggesting instead to mine fissionable elements, although the quantity was insufficient for their needs.
The humans decided to leave Mantilis and try to find flora and fauna lower down in the fjord, requesting power supplies from the Caeliar. Sedín was going to describe the folly of their attempts, but Lerxst cut off her reply through the gestalt. After Lerxst agreed to their request and the humans departed, Sedín questioned the wisdom of the decision, as it would jeopardize their own survival.
After the humans departed, the Caeliar took on non-physical forms and began to lose sensations of feeling and time. As they scrambled to find alternatives to ensure their survival, Sedín was again pessimistic of suggestions, including dismissing the possibility of a symbiotic relationship with a biological host. After the suggestion of consolidation and merging with the humans was made, Lerxst agreed to present the option to them, which the humans refused.
The Caeliar consolidated their numbers from twelve to four, until only Sedín, Lexrst, Denblas, and Ghyllac survived. Sedín sensed their impending demise, and when Denblas gave up his life for the others, only she and Lexrst gained strength from it; Ghyllac's catoms had lost the ability for thought and memory, and Sedín argued strongly for the action to take his catoms to strengthen their own lives. Lexrst was distraught by the thought, but Sedín's threat to take them all for herself forced him to perform the task and share the energy.
Later, as their own memories continued to fade, Lexrst questioned if Sedín and he were ever friends, and when they begin to fade, would she consolidate him without thought and compassion. Sedín thought moral considerations should be secondary to their continued existence, and resolved to merge with the humans even if they did not desire it.
After the humans were unsuccessful in finding any useful materials along the coast, the few remaining alive returned to Mantilis, where Karl Graylock was the first to be inhabited by the barely coherent Sedín. His last thought was that he would not become a cyborg, and hence the name for the Borg was created. She then merged with Gage Pembleton and Kiona Thayer; she deemed the female and engineer most valuable, and had them feed on Pembleton for sustenance and then gave them the first command - sleep.
Sedín remained the driving force behind the Borg Collective, assimilating new species and new technology into her gestalt and relentlessly seeking the Omega molecule, the ultimate source of energy, and ruling through her various Borg Queens. The most resistant to the Collective would be the United Federation of Planets, which saw an all-out invasion in 2381, intent on destroying them and their local allies. Former Caliear captive Erika Hernandez escaped New Erigol aboard the USS Titan and made contact with the Borg, realizing they were the same. After drawing Axion to the Azure Nebula to lure the Borg fleet in with their Omega molecule generator then opening subspace tunnels to draw the Borg vessels back, Erika acted as a conduit between the two, through which Inyx and Ordemo Nordal found Sedín's violent scavenger shreds. The Caeliar dissolved her consciousness and freed the trillions of Borg drones, incorporating them into their low numbers to begin the process of rebuilding their gestalt and continuing the Great Work. (ST - Destiny novel: Lost Souls)