An idea has formed, taking bits from all the suggestions, and a bit from a discussion with my brother.
Bysipsis - The Vassal Worm, god of knowledge, water, exploration, risk, fortune, wealth, the unknown, inspiration, art, healing, innovation, strategy, darkness, cowardice. Depicted in a subervient position to the other deities, Bysipsis is often portrayed as a serpent, leech, worm, eel, dragon or disfigured human. Associated with quills, scrolls, curved geometric shapes and serpentine designs.
Bysipsis is a being from the dark, born beyond the light of the gods, a member of the demonic pantheon. Doctrine holds that Bysipsis swore an oath of allegience to Lumidium after being bested in combat by the sun god, pledging eternal service in exchange for mercy. The serpent god lurks in the darkness outside Lumidium's light and watches over mortals who choose to stray out of the sun's vigil. At times Bysipsis sneaks into the domains of the other demon gods to spy on their plans and steal their secrets in order to spirit them back to Lumidium in hopes of currying his favour. The serpent has a terrible fear of danger, surpassed only by it's greed, coveting and risking much in order to hoard objects of value and beauty and it's fawning desire for Lumidium's respect.
Temples honouring Bysipsis often serve as hospices, tending to the sick and the lame, and providing shelter to vagrants and the homeless who are collectively seen as mortal reflections of the deity. While many serpentine or wormlike animals are considered somewhat sacred by the church, only leeches are kept in any significant number on temple grounds, though they are treated more as a disposable tool of the faith than a holy being in their own right. Larger temples often serve as libraries, collecting, translating and preserving knowledge to a greater extent than churches built to the other deities. The very largest temples serve as acadamies, training wealthy students in a range of useful skills.
Lumidium is begrudgingly revered by sailors, merchants and explorers. Many resent the associations with the poor and the sick that come with the faith, with some trying to create gated churches that do not act as hospices, placing greater emphasis on the artisanal aspects of Anaxus, the courage of Lumidium or turning to foreign gods of commerce and travel.
Festivals honouring Bysipsis traditionally involve pardoning criminals, tending to the sick and launching seafaring vessels, and are celebrated during the new moon.