This is epic fantasy, first in a series. All of them will have similar titles, starting with the word "Until." For example, the second book is called: Until All Bonds are Broken. So the title design needs to be something that can be consistent across the series.
I've uploaded some sample covers that I like.
The following are random brainstorms of cover ideas. None of these are necessary, but will perhaps provide guidance, mood, etc., toward a final concept.
One cover possibility could be the two protagonists. Here are their descriptions:
Marshal: Male, 18 years old. Caucasian. Light brown hair and eyes. Wears quasi-Roman peasant garb. His face is heavily scarred, as if burning ropes had wrapped around his head and left their mark. Looks serious. Mouth closed. He can be holding a sword (broadsword, though narrow). Either his sword or his free hand can be vibrating to indicate his magical power.
Seri: Female, late teens. Olive skin tone (Middle-eastern). Dark brown hair. Brown eyes, with a noticeable sparkle (star) in her left eye. Wearing mage robes, either light orange or mid-tone blue. Can look playful, smiling. Perhaps holding out her hand, palm up, with multi-colored beams of light exploding from it. (In fact, the hand with the light beams could be a cover all to itself.)
Possibility #2: Landscape of open plains. A huge city of white stone and polished wood floats in mid-air, about 7-10 feet above the ground. A solitary figure kneels on the ground, facing the city, back to the viewer.
Possibility #3: Combat scene. Marshal (described above) facing off with an assassin clad in gray tattered clothes, but wielding a shining blade. The assassin’s skin looks diseased, hairless, peeling (somewhat like leprosy). Setting can be forest. Shallow stream flowing down an incline. Spring leaves on trees. At least one with white flower blossoms.
Possibility #4: Mood/landscape. Dark. Ruined land, fallen trees, craters, etc. But massive star field above, including stars of varying colors and brightness (some huge). No moon. Maybe silhouette of someone looking up at the stars (holding a sword? With stars reflected in it?).