As we understand the sources, Umaril the Unfeathered was an Ayleid sorcer-king who challenged and was defeated by Pelinal Whitestrake in the wars of the Alessian Rebellion. However, due to an association with Meridia, his spirit survived in the Colored Rooms, from whence he returned following the Oblivion Crisis, to attack Cyrodiil when the empire of man was in a weakened state. Umaril was defeated by the champion of Cyrodiil, who restored a lost knightly order dedicated to the Nine Divines, and mantled the Shezarine in order to destroy Umaril’s spirit once and for all.
Why Meridia? Meridia is a daedra about whom we know vanishingly little: she is associated with the http://www.imperial-library.info/content/morrowind-book-daedra, abhors the undead, and is http://www.imperial-library.info/content/imperial-census-daedra-lords. It also seems that she is (or was) associated with the Ayleids. She agreed to the pact with Umaril, after all; moreover, http://www.imperial-library.info/content/oblivion-song-pelinal-3 refers to White-Gold Tower as: “where the Ayleids had made pact with the Aurorans of Meridia, and summoned them, and appointed the terrible and golden-hued “half-elf” Umaril and Unfeathered as their champion.” White-Gold tower was the center of the Ayleid Empire: the fact that the pact with Meridia was made there suggests she had a prominent place in their pantheon.
Meridia is associated with living energy and light (hence the sun, hence Magnus). Our understanding of magicka as coming from Aetherius through the sun and stars appears to be of http://www.imperial-library.info/content/oblivion-magic-sky. In fact, Aetherial magic in general seems to be associated with Ayleids: notice how even today, mer typically are much more adept in the magical arts than men. Men, in contrast, are associated with nature-based powers: consider the importance of Kyne to the Nords. Thus, a pact with Meridia makes perfect sense.
Following his defeat by Pelinal, Umaril’s spirit was banished to Meridia’s realm. This appears to suggest that Umaril’s immortality is due to his pact with Meridia. However, according to http://www.imperial-library.info/content/oblivion-song-pelinal-3, “Umaril had the blood of the ‘ada and would never know death.” http://www.imperial-library.info/content/oblivion-song-pelinal-7 also reports that before his battle with Pelinal, “the half-elf showed himself] bathed in [Meridian light]… and he listed his bloodline in the Ayleidoon and spoke of his father, a god of the [previous kalpa’s] World-River.”
So which “god of the previous kalpa,” is the father of Umaril? Well, http://www.imperial-library.info/content/oblivion-song-pelinal-6 quotes Pelinal in a fit of divine rage as declaring: “O Aka, for our shared madness I do this! I watch you watching me watching back! Umaril dares call us out, for that is how we made him!”
Here’s where it gets interesting. Akatosh doesn’t exist yet. Akatosh, the chief god of the Cyrodiilic pantheon, is an amalgamation of the http://www.imperial-library.info/content/morrowind-varieties-faith-empire, and thus only originates after Alessia overthrows the Aylieds and creates the Eight Divines. The most obvious resolution of this puzzle is to recall the fact that Pelinal is from the future. But if that is the case, it raises some terrifying implications.
There’s additional evidence that this terrifying implication is, in fact, the truth. http://www.imperial-library.info/content/oblivion-song-pelinal-1 testifies to men giving thanks to the Eight for Pelinal, which like Akatosh, do not yet exist as a pantheon. http://www.imperial-library.info/content/attack-chapel-anvil. “Our ancient enemy has returned, from the deeps of time, to exact his revenge upon the gods. The gods, the Eight Attendants of Saint Pelinal when he smote Umaril the Unfeathered.”
Both the Song of Pelinal and the Prophet himself explicitly state that Pelinal went to do battle with Umaril alone. So how could he have Eight Attendants? There seem to be two answers, and I think each is correct. One answer is that the Eight Attendants are represented by the Pelinal’s weapons and armor, which come from the future, and were granted to him by the Eight Divines. The second answer is that there were others present when Pelinal defeated Umaril. They cut him into http://www.imperial-library.info/content/oblivion-song-pelinal-7.
That number is no accident. According to http://www.imperial-library.info/content/oblivion-adabal, Pelinal reported to Morihaus: “Our enemies of undone me, and spread by body into hiding. In mockery of divine purpose, the Ayleids cut me into eights, for they are obsessed with this number.” So, is the mythical significant of Eight a merish or mannish creation? Again, the implications are terrifying.
"... and left you to gather sinew with my other half, who will bring light thereby to that mortal idea that brings [the Gods] great joy, that is, freedom, which even the Heavens do not truly know, [which is] why our Father, the... [Text lost]... in those first [days/spirits/swirls] before Convention... that which we echoed in our earthly madness. [Let us] now take you Up. We will [show] our true faces... [which eat] one another in amnesia each Age."