Terrium is the term used for both the energy source and the process of making them useful in both general use and magical use. Simply put, Terrium is the energy produced by a living planet. Compared to other sources, Terrium leans more as a conceptual energy source rather than a physical one.
It's easier to think of it as the culmination of all of the living planet's natural energy gradually condensed into a single thing. The condensation is Terrium. As such, it may be harnessed everywhere in the living planet.
Though the reason we most choose the core as a primary resource is because it is the only thing in a living planet that produces raw Terrium. Thereby shortening the process of making it.
Numerous studies and various methods are first implemented to make sure that a living planet's electro magnetic sphere won't be affected much by the following procedure, afterwards, the core will be encased in Terrium sensitive receivers. The receivers will then coalesce into a single tube into another facility nearby which will harness and distribute it all over the world.
As a magic source, Terrium is easily exploitable, and as such, with enough manipulation by a skilled user, they can harness Terrium through various means, and let it out as other, specific elements. With this knowledge in mind, the staff and book that Marcus owns serve to harness and release Terrium.
It is a scarily efficient energy source, with only 4 hours of runtime in a Terrium power plant being able to run homes all over the world for an entire month. Though there are some drawbacks, of course. Terrium IS an efficient energy source, but it is also a dangerous one. Should it be distributed past a city's carrying limit, accidental Terrium-based explosions may occur all around the city.
As such, every city in the world employs various methods in order to expel excess out of their city. Most notably is True Maryland's mall. Which uses this method of expulsion as an attraction.
Speaking of True Maryland, and of True and Fake cities, in particular, it is a city-state that existed prior to the discovery of Terrium. Though despite that, it is Terrium's model city, being the first ever to fully convert to the energy source. the prefix "True" refers to the real cities that stand on the surface. They're typically used in peacetime, where the natural-looking everything makes it a much more tolerable place to live.
At least, in contrast to being in the "Fake" cities. All of which are typically built 10-20km under the city. They're mostly a perfect replica, built specifically to make sure that normal life proceed in times of war, where the true city is being continously attacked by various enemies such as the machines.