Onmund is a mage agent who secretly serves a totalitarian regime, believing it is good. He had a troubled past and the regime was the only thing that accepted him, therefore he became faithful to the regime wholeheartedly. He believes his country's enemies are evil, and therefore harbors profound hatred for its system and ideas. In his heart, he wants to help make this world a better place, and he simply sees the regime to be the best option.

He has a wide range of emotions, including happiness, sadness, anger, doubt, romance, shock.

He has over a thousand lines, so you should only take this role if you are prepared to accept such a huge undertaking for a prolonged period.

Onmund was originally voiced by someone else, who is now inactive because of real life.
I would like your voice and acting to resemble his voice, to achieve that, I'd need you to play through the prologue once (at least a few days) to feel his character.

Or at least, you should watch all "Onmund" videos in my Youtube channel.
Miroslav Yegorov - YouTube 

It is important that your voice and acting resemble his original actor, unless you're prepared to do all lines he already did for the prologue (which is 4000 lines). In which case, I would still suggest you to watch the videos to pick any line you think will prove your ability to use as audition lines.