Ioun is the goddess of magic and knowledge. While the other gods spent the early years of the world exploring the physical planes, she explored the Astral Sea and eventually discovered The Source. When Vecna began his campaign to seize control of it, she dedicated her endless life to defending it from him.
Due to her detachment from the world, Ioun has very few worshippers, but there are a number of scholars who worship her, such as Moda.
Ioun's most notable achievement may be the creation of the Common tongue, back when she had uncontested control of The Source. She created a language which could be produced by most species' vocal apparatus, and granted knowledge of it to all sentients alive at that time as an instantaneous effect. Although the knowledge was not magically granted to future beings, the Common tongue's obvious utility led to it being widely passed down through the ages (and with less regional/temporal deviation than most other languages). This allowed sapients of all nearly all species and nations to communicate with each other, massively aiding the spread of knowledge throughout the world.