Umbra is one of the five affinities, the colours of power that leak into the Hallow through the imperfect Covenant. It is the affinity of death and its aftermath, and Deopa, the Deep holds it.
On the ring of affinities, each affinity feeds the next. Dreams die, so Reverie feeds Umbra. The dead burn, so Umbra feeds Pyre.
A Sealborn whose core wakes in Umbra can master its spells and reach the complex spells of its neighbours. Lumen and Bloom lie across the ring and repel it; from them, such a mage learns only the simpler spells, and casting against the grain costs more, fails more easily and lands weaker. The exact prices belong to the ruleset.
Since the Breaking, the natives' dead have lodged at the threshold of the Deep, pressed against the underside of the world. Where that threshold comes close to the ground, Umbra can reach them. They are the restless dead, and they rise on life borrowed from the living. Every army of them is paid for in people.