The caster summons an unkindness of ravens and sends them into a distant province to feast upon the newly dead. The ravens consume the rotting corpses and return to be slaughtered for the raw death essence they then contain.

Raven Feast is an 4 ritual at Conjuration 5 that costs 3. The spell converts corpses to death gems and can be cast at both friendly and hostile provinces. The spell yields $\frac{1}{2}\left(1+\sqrt{\mathrm{corpses}}\right)$ death gems. For instance, 9 corpses yield 2 death gems, 121 corpses yield 6 gems and you'd need a full 361 corpses for 10 gems.

The ravens are not actually summoned into the game.

If there's corpses around, such as if you've scouted or taken part of a big battle, Raven Feast is an efficient way of converting air gems to death gems. Its main drawback is the required path to cast it, which few nations have access to, and such strong mages often have even more valuable uses of their turns.