Angvimar Luthenian was a legendary dark elven wizard, who lived before and in the early years of Kingdom of Borealis. Known to be the most powerful elven wizard to ever defect from the Eldar Shores and abandon its genocidal war against Gwynwald wood elves, his arcane, scientific and civic studies were fundamental to the foundation of the Eldritch Circle. He is most famous for formulating the Angvimar Theorem, which has existential significance for modern theology and arcane arts.
Life and death
A high-born dark elf, Angvimar was the only son of the key hierarch of the Eldar Shores high command and received superb education, which, together with his family's inherent magical talent, made him unsurpassed both in arcane combat and scientific argument. Despite that and, allegedly, being privy to the secret knowledge that justified the eternal elven war, he was its stark opponent. This earned him enough enemies among the high command to eventually consider defecting from his homeland.
Angvimar was granted asylum in Southern lands of Borealis and settled in a tower in Voegelwald, in Volgen, conducting extensive research in arcane arts and, surprisingly, natural science, which most of wizard and nobles at the time (and to this day) considered low-brow and unworthy of attention, compared to matters and studies spiritual, such as theology and magical theory.
Extensive use and knowledge of the scientific method, as well as long lifespan, allowed Angvimar to conduct research which led him to formulate the Angvimar theorem. Having witnessed the monstrous atrocities committed by elven magic against armies of Everard Boreale first hand, he foresaw the brutal backlash Thessean magic users were about to face from commoners and those in power alike and advocated regulation of magic on Thessea, writing down the first treatises to be later used as a framework for the Eldritch Circle's creation.
Shortly after coming up with the Angvimar theorem, Angvimar allegedly started to fade away, having lost the will to carry on in light of his research's inevitable, as he saw it, conclusion. During one of the harshest phases of witch hunts and pogroms in Borealis, his tower in Ransewald was surrounded by an angry mob, demanding to see the wizard. He descended to see them himself and was promptly torn apart by them. Angvimar offered no resistance, saying, as the tale goes, that "long gone's the time of this Realm, when your barbaric antiques might have offended me". His violent death prompted the remaining wizards of Thessea to commence talks with the Borean Crown, which ultimately resulted in the creation of the Eldritch Circle.
The Angvimar theorem
Proposition 1: There has been observed a decline of raw energy manipulation potency of a single magic-user over the course of 15, 50, 100 and 250 years;
Proposition 2: Proposition 1 cannot be explained by decline of the magic-user's individual powers or mental capacity; [Angvimar was a dark elf and was not even that old, around ~500 years old, when he died, being reported by his contemporaries to increase his acumen and power almost yearly]
Proposition 3: The decline can be observed even after proper adjustments for ley-line proximity have been made, making it impossible to explain it by reference to location and external energy influencers;
Conclusion and theorem: there exists a proposed accumulated amount X of raw magical material subject to mortal manipulation in the world. With flow of time, magic users material manipulation output falls. Therefore, it is highly likely that the world's raw magical material (assuming existence of a source) is not replenished at a sustainable rate OR (assuming non-existence of a source) have never had a capability to replenish in a first place.
Summary: the amount of arcane magic in the world is finite and is not replenished over-time at a sufficient rate (if at all).