Reverie is one of the five affinities, the colours of power that leak into the Hallow through the imperfect Covenant. It is the affinity of dream, illusion and mind, and Aefenna, the Lamplighter holds it.
On the ring of affinities, each affinity feeds the next. Order dreams, so Lumen feeds Reverie. Dreams die, so Reverie feeds Umbra. The ring describes how the leak cycles, not a road every soul must walk.
A core takes its affinity when its bearer wakes, and keeps it all Turning. A Sealborn whose core wakes in Reverie can master its spells and reach the complex spells of its neighbours. Pyre and Bloom lie across the ring and repel it; from them, such a mage learns only the simpler spells. Casting against the grain costs more, fails more easily and lands weaker. The exact prices belong to the ruleset.
Cultivators of Reverie have their own reason for loving it. Every foreign soul wakes remembering another life, they say, so every Sealborn already carries a whole world that now exists only in the mind.