Ghyllac was assigned to work on the city-ship Mantilis while it was on the planet Erigol. A feedback pulse destabilized the planet and necessitated Mantilis' departure in order to protect several human Earth Starfleet officers and crew who were attempting a hostile action to allow the Columbia to escape.
The ship traveled a subspace tunnel, throwing it through space and time from the Azure Nebula in 2156 to Arehaz in 4527 BCE. After the landing, Ghyllac showed Gage Pembleton and Karl Graylock into their de facto chambers, and later questioned Lerxst after the humans were offered batteries and resources from Mantilis' low reserves. Ghyllac pointed out they required the energy to survive, but Lerxst reminded him that the humans did as well.
Along with the other eleven Caeliar aboard, Ghyllac began to lose cohesiveness away from the quantum field of their planet, becoming one of them to forget what simple things like twilight were. As they became concerned for their existence, fellow Caeliar Felef considered consolidation of their catoms; Liaudi and Meddex were concerned how they would decide who would surrender their energy to the gestalt. Ashlok had already made an analysis of how to achieve greatest return from the least amount of sacrifice, and determined that any sacrifice would inevitably feel arbitrary, and it would be best to allow consciences to guide rather than a tyranny of numbers.
Dyrrem and Narus suggested that they could feed on the local fauna like the humans, but Sedín pointed out it would not be possible to cross back into the organic existence, and Ghyllac realized they would not be able to guarantee sufficient neuroelectric activity without a sentient lifeform. Sedín wished to merge with the humans, and Lerxst agreed to present the idea to them, but Ghyllac, even the cynic, inquired what would happen if they refused.
The Caeliar's numbers eventually dwindled to four - Lexrst, Sedín, Ghyllac, and Denblas, all of whom lost many memories. Ghyllac found that he could no longer recall what their species was called. Denblas released his catoms to boost the others, but Ghyllac reaped no benefits, his mind having trapped itself into a dark spiral of confusion and discovery. Sedín threatened to absorb all of his catoms, but Lexrst participated and shared in his sustaining energy. Lexrst asked if Sedín would remember him fondly as friends when one of them was absorbed, unlike the cold reality they faced Ghyllac's final moments with. (ST - Destiny novel: Lost Souls)