Lumen is one of the five affinities, the colours of power that leak into the Hallow through the imperfect Covenant. It is the affinity of order, law, light and protection, and Daegred, the Sunward holds it.
On the ring of affinities, each affinity feeds the next. Growth orders itself, so Bloom feeds Lumen. Order dreams, so Lumen feeds Reverie. Cultivators call this ring the stalled wheel. It describes how the leak cycles; it is not a road every soul must walk.
A Sealborn whose core wakes in Lumen can master its spells and reach the complex spells of its two neighbours. Umbra and Pyre lie across the ring and repel it; from them, such a mage learns only the simpler spells. Power forced against its grain curdles, so casting outside one's own affinity costs more, fails more easily and lands weaker. The exact prices belong to the ruleset.
Ysme of Lumen, who lent her protection to the Flint lineage in the Age of the Given Word, cultivated this affinity.