Thaletas and a number of others were dispatched to the Silver Islands to combat the Athenian armies of Podarkes. Along the way their fleet fell into a trap set up by the Cult of Kosmos, sinking their three ships and killing several of Thaletas' companions, including generals. Because of this, Thaletas assumed the role of the polemarch for the remaining Spartans as they set up their camp on Mykonos' north-eastern coast. With the ship also sunk Thaletas' prized heirloom, the helmet of his grandfather, who had fought and fallen with King Leonidas at Thermopylai.
While Thaletas had left for the Silver Islands in order to merely strike a blow against the Athenians, meeting Kyra changed that, according to Thaletas. In his own words, seeing Kyra made him sure he'd never leave the islands. He thought this to be love, though meeting Kassandra made him question it all.
Behind the scenes
Thaletas is a romanceable character in Assassin's Creed: Odyssey. However, doing so will disrupt his relationship with Kyra.
If the rebel Kasos is spared in The Sokratic Method, the extremist will assassinate Thaletas in the celebration. However, he can be spared if Kassandra chose to leave the party after saying goodbye instead of giving a speech.