Every mortal on Nirn consists at least of four essences: health, stamina, magicka and physical body with a soul to animate the mechanism. Galerion showed us it is possible to separate them.

To cast a spell a spellcaster must sacrifice his essence the same way a warrior swinging an axe sacrifices his stamina. What essence to sacrifice is completely another question. A shaman dancing with a tambourine exhausts his stamina, a mage speaking out an incantation exhausts his magicka, but they both might have a similar result of their spellcasting. It's like those Expunge and Purge skills - they both remove negative effects from the caster with some slight differences, but it takes a Purge caster to sacrifice a part of his magicka, while a necromancer sacrifices his health to cast the Expunge (think of a necromancer sacrificing some of his blood with a knife in a dark ritual). The same way it takes stamina to draw a Circle of Protection on the ground, or, say, it takes nothing but time to Invoke the Rites of Moawita or to meditate. Morphs are rituals performed to achieve a certain result, so different morphs of, say, a Blastbones incantation result in different skeletons you raise, though they are still skeletons.

Have you read the book called On the Preparation of the Corpse with it's volumes 2 and 3 describing what it takes to raise a simple zombie? I guess it takes more of stamina there, don't you agree with me on that? Anyway, there are so many books on magic that describe the process..

The Four Suitors of Benitah: "Yakin taught him the spell to fortify his strength. It took him some time to master it, visualizing magicka streaming through his body, pumping through the very fibers of his muscles for a time, giving him strength far beyond the puny power nature had intended. When Oin met Horath on the street of Gnisis, he cast the spell and challenged him to a duel of strength."
The Power of the Elements: "Elemental magic wielded, Elemental thoughts displayed. Havoc wrought as if for sport, Efforts to impress fall short. I'll merely use a blade. Seeking study, wanting learning, Recklessly aroused my rage!".
Breathing Water: "the spells of Alteration are all about uncommon sense. The infinite possibilities, breaking the sky, swallowing space, dancing with time, setting ice on fire, believing that the unreal may become real.".

I won't continue their list - I hope you get the point, OP.

Bruccius wrote: »

Magicka is something each mortal has access to thanks to Magnus (the sun), who left behind the essence of Magicka when he abandoned the creation of Nirn. (Ot at least, something along those lines).

Each of the stars in TES are these ''holes'' to Aetherius, where numerous powerful entities who abandoned the creation of Mundus left the world. Magnus was one of the largest and most powerful of these entities.

Again this typical delusion and propaganda of an Eight Divines worshipper !.. Why do you, the Eight Apostasies worshippers, always force these myths as if it is the only truth and something accepted by everybody? What if I say you the stars were placed by Ruptga to guide the spirits to the Far Shores and magicka essence is created by Satakal, the God of Everything? Where would your Magnus be left then? Though the Sixth Redguard Maxim states "Fear not the unbelievers, for believer and unbeliever alike shall be eaten by the Serpent God" and though I'm not a Redguard and do not follow Hel Shen She Ru, I do not force anyone to believe those tales as the heretics like you do .