Magic

Magic is a divine, mystical force that permeates the cosmos. The true nature of it is unknown, but it is probably a remnant of the Old World - a chaotic universe in which matter was continuously formed and destroyed in an infinite cycle. It is probably the source of the Elder Gods’ power.
Some creatures have the power to utilise this force to manipulate reality and matter in what we now call spellcasting. It is incredibly taxing on the user, and so mortals can only use it a few times a day.
More magical power can be obtained through a ‘blood sacrifice’, a ritual to weaken the user physically but strengthen them magically. More people together can achieve a higher magical power output, to achieve what is unavailable for mortals separately. However, this typically also requires a blood sacrifice.
Beings with higher magical power, such as deities or Gods, have been known to be able to gift or trade this ability for services or in exchange for their continued worship. This form of magic is called divine magic, as opposed to arcane magic.
Magic often takes years of study and practice to master, but only a few are gifted with it. Other ways magical power can be obtained include through a magical bloodline, or being raised in a high-magic location.