The First Satan: Rise of Yeqon (The A'vean Chronicles) by GR Thomas

Bonus Content: Prologue and Chapter 1 of Awaken, book 1 of The A'vean Chronicles.

At the dawn of civilisation, angels roamed the Earth.
Within shadows and dreams, these great warriors of A'vean were the silent caretakers of humanity, watching the first civilisations pull themselves from the cesspit of evolution.
I'el, the creator of all things, set one directive… do not interfere.
Yeqon, sentenced to Earth as punishment for his misdeeds elsewhere, resented this deployment. Mindless eons of gently nudging humanity in the right direction set his mind to wander on the past and future glory; of war and power. Cowering in the shadows at the behest of I'el set a flame of resentment alight within him.
Yet, a loyal warrior, Yeqon remained committed to the task when he yearned to travel the universe, to fight, to love... until I'el pushed that loyalty too far.
A cord was struck against Yeqon and the Watchers he commanded. An event so devastating that it cleaved a wedge through them and stoked that fire. This upheaval stirred the beginnings of the greatest of wars…
and birthed the First Satan.