It's a fully voice-acted quest mod focussing on Arkay. It features well written dialogue and a great quest. Anyway, here's the original information from Nexus:
So the citizens of Tamriel have been taught about their God of Life and Death eon after eon. It is one of the Ten commands, and one of the building blocks upon which societies and civilizations have been built.
But what if Arkay, the embodiment of the eternal life cycle, had perished long ago? What if a secretive underground society - rumored to have unmatched magicka - fiercely guarded secrets such as this? What impact would it have on the Imperial Cult religion that has flourished and brought order to a once chaotic world?
And what if you were allowed to discover this awful secret? What would you do with this divine knowledge?
This opportunity awaits you in: "Et In Arkay Ego," an expansive mod for the Elder Scrolls IV: Oblivion.
...
As an adventurer in Tamriel, you come across a collection of documents that hint at an amazing secret - that the God of Life is dead! Following an interesting trail of clues and quests, you will come across an underground society thought to have perished thousands of years ago. From the idyllic-minded Shepherds of Arkay, to the lore-minded Brothers of the Underground Stream, to a bard humorously singing your praises (or shortcomings) Et in Arkay Ego is a mod unlike any other.
This mod will feature over five hundred new voice files, an original quest and history, over thirty new NPC's (non player characters), twenty books, ten locations, items, images, meshes, textures, spells, and music. Players will experience a whole new side of the "Elder Scrolls IV: Oblivion" on an epic adventure into the soul of Tamriel. Do you dare seek the knowledge and power of a God?
Why no link? Well, there are two versions of the mod: "Et in Arkay Ego" and "Et in Arkay Ego - The Spiritus Version". The second one contains non-sixual nudity and that's why I don't know if I'm allowed to post a link to the whole mod as they are both uploaded to the same location @nexus.
Anyway, just search nexus for "Et in Arkay Ego". Or search google for "Et in Arkay Ego" and you'll get the link to the official website