Their spawning grounds were destroyed when aliens colonised their home planet. As a result, they set off to find new spawning grounds. The Calaxi's physiology made them the perfect incubators for their seeding crystals, but the Spherions had no idea that they were sentient and so when the Calaxi fled Malleon, the Spherions followed.
Inert fragments of Spherion crystals also existed in a pocket of unreality, which allowed this bubble dimension to link itself to the Silversmith's Mirrors, turning the Mirrors into portals between the pocket dimension and 19th century Edinburgh. The link was strengthened by the Mirror Slversmith, who fed the Spherions psychic energy stolen from his human prisoners; processing the energy, the entrapped Spherion crystals allowed the Silversmith to sustain the existence of Mirror Edinburgh as well as that of the portals. (COMIC: The Silvering)