Phage started out as a three kilometre long asteroid. It was converted into a habitat after resource mining, and then into an interstellar vessel. By the late 19th century CE, after 9000 years of travel, it had grown to over seventy kilometres in length and resembled a cairn. The surface was dotted with a myriad of structures and facilities, some no longer functional or in good repair, accrued through the ages.
The original Phage asteroid was in one of the first systems explored by one of the species that would later form the Culture. Once resource mining was complete the asteroid was spun to produce artificial gravity and turned into a habitat.
Political tensions in c. 7000 BCE induced the asteroid to be converted into an interstellar ship and leave the system. It was equipped with fusion-based steam rockets and ion engines, and armed with re-purposed signal lasers and mass drivers. Phage was involved in some combat and after a few years it was recognized as a sentient entity by its inhabitants and joined the nascent Culture.
It travelled from system to system concentrating first on trade and manufacturing. Universal productive capability resulting from technological advances induced a change toward a more cultural and educational role.
Within a thousand years of becoming a habitat, Phage grew into a thirty kilometre long cone with the front half of the original asteroid poking out the front. Growth slowed after that and by the late 19th century CE it was over 70 kilometres long with a population of 150 million.