Thalassa is an Ocean World in the Ultima Segmentum. Originally a Forge World during the Dark Age of the Imperium, a catastrophic accident caused a rapid, uncontrollable rise in sea levels. The unfortunate population either took to the waves in ramshackle craft or provided the nutrition that kickstarted a sudden explosion in marine life, heralding the geological development of a planet size ocean.
Rediscovered during the Great Crusade, Thalassa’s drastically reduced population had developed a trade based society centered around huge city-state flotillas. Their once high tech society reduced to seasonal migration and intermittent warfare. Eventually Thalassa became the homeworld of the Sauroters Chapter whose fortress monastery occupies the only land mass on the planet.
Located on the equator, Halia is the summit of a vast mountain, the bulk of which is underwater. The Sauroters maintain a fortress monastery there where aspirants may present themselves, having first solo-circumnavigated the planet as proof of their worthiness. Those not claimed by the dwellers of the deep oceans, nor driven mad by the unrelenting sun, are ready to begin their transformation into an Astartes.
As the Imperial Truth gave way to the Imperial Cult, so too did the Thalassan’s interregnum sun worship come to be replaced with an unwavering belief in the Emperor of Mankind. Represented now by the Vergina Sun. Symbol of the Sauroters Chapter and holy Terra’s ancient Agean past.