Just one dark mo-fo who is searching for Lia. (spoiler alert, this is really Malthael - Archangel of Wisdom - later known as the Reaper of Souls) Lore dump: https://diablo.fandom.com/wiki/Malthael 

Voice from Diablo 3: https://www.youtube.com/watch?v=JUWoj9evVWM 

Here's what's happening: Malthael found out about Lia's existance, due to Izual's Trial in the High Heavens and later banishment to the Burning Hells. Mephisto may have spilled the beans to him that he had a family, and that the daughter of the betrayer still lives. So Malthael is hunting her. 

What Lia is thinking, this is somebody who knows her secret, she accidentally killed a human she thought was a demon, and then covered it up. It was during her cover up, that many of her troops from the Watch were ambushed and killed due to her not being with her unit. She holds onto the guilt from that day and thinks this darkness is what is coming for her.

Voice Notes: It's a minor role, so have fun with it. Sound evil and threatening for sure. Malthael will play a much larger role later in the story, so you may be recalled to reprise the role when the story comes to an end.