The Rules of Magic

White Mystia, Black Mystia, Blood Mystia, and Psi Mystia do have their limits. And consequences. Such as Blood Mystia's necessary ingredient: blood. This blood must come from a live source, as once the body dies, the Mystia in their blood fades quickly.
Psi Mystia users face the same limits in their powers as White Mystia users do, in that the Mystia that flows within their veins is very limited. Using too much of it will leave the user drained for some time. This is why White Mystia users augment their powers with crystals, which can store a small amount of Mystia as well as focus the Magician's existing powers. But crystals are limited in this day and age. Over-mining of crystals through out the Ages has left us with very few remaining places from which to mine. This has left the world in a very precarious position. In only a matter of years, the society will come under a very different kind of threat: the threat of waning resources.
Psi Mystia users do not have the luxury of using crystals to augment their powers at all. Instead they have to rely entirely upon their own power, which can be deadly to the user in the case of a certain ability: Psychic Burst, which immediately drains the user of all of their Psi Mystia and unleashes it in an explosion of concentrated energy, which could kill the user if they are not careful.
However, Black Mystia users face very different consequences. Their power does not come from the Flow, and instead comes from souls they have stolen from victims, a practice which carries many perks, such as long life, but as many flaws. The more Black Mystia one uses, the more their body will break and decompose under the pressure. No humanoid's body was designed to withstand the effects of Black Mystia, as is clearly shown in the case of one very infamous individual: Evilon.
For these reasons, Magicians and Telepaths alike choose to summon creatures to help augment their powers further. Telepaths can summon forth elemental incorporeal beings called Elemental Spirits. Magicians of all Colleges can summon animals using the Conjuration School or, more commonly, Mystimon upon whom they have imprinted. But the very act of summoning also uses a large amount of Mystia, and for Magicians summoning Mystimon, this also carries a bit of risk. If an imprinted Mystimon is killed in battle, its controller will also take damage.
Magic has its limits. We must learn to work within those limits. In this case, magic's biggest limit is its finiteness. There is an end to so-called endless power. Nothing in this world or in any other is infinite. Unless you live in the Sacred Nexus. But that's another story.