Borost:Moldenteft's Law

From Bestiary of the Hypogriph

Moldenteft's Law, also known as the Fundamental Principle of Magic, the Principle of Equivalency, and the Fundamental Principle of Borost:Reagent theory, is a magical principle that states, roughly, that in the world of the arcane, "nothing comes from nothing."

When drawing energy from the Skeleton of the World, something of equivalence -- a reagent of suitable proportions -- must be consumed in exchange.