The Battlestar Galactica actor will play a "brilliant" and "charismatic" professor of religious studies in the new run.
Olmos told IGN: "I think that whether it's the fifth season or the first season or the 20th season, it all depends on the storyline. And this is the darkest season of them all. I've watched the show from the beginning and this is the darkest one."
The star also revealed more details about his role on the Showtime drama, claiming that his character is "far from being the criminal".
"Dexter is the real criminal," he said. "You'll have to see the show. As soon as you see [the season] you'll understand what it is. It's probably the most twists you'll ever get in your life."
Dexter star Michael C Hall recently confirmed that his serial killer character will contemplate issues of faith and spirituality in the new season as he continues to raise young son Harrison.
Dexter will return to Showtime on October 2.
> Dexter Brea Grant interview: 'I don't normally get to play the hot girl'
Watch a trailer for the new season of Dexter below: