Marked

This dark portal fantasy introduces Detective Dana Rohan, an officer who solves crimes using the Mark that allows her to travel to alternate pasts and futures. Detective Dana Rohan has an excellent arrest and conviction rate. But even her partner doesn't know the real reason why. All her life Dana has borne a Mark of unknown origin that she's kept secret. A Mark that allows her to walk into alternate pasts and futures.
A Mark that allows her to go back and see any crime as it's being committed. But the life she's carefully built around this secret ability begins to crumble when she's assaulted by a ragged old man. He babbles an incoherent warning that "the Shadows are coming, " right before he is killed by an armored monstrosity out of another century. The armored attacker vanishes, leaving the old man to die in Dana's arms, and she realizes that he bears the same Mark she does.
Soon Dana finds herself hunted by Shadows coming from out of Chaos. She must flee through a host of alternate worlds as she finds out the true meaning of the Mark on her skin, and why someone wants to kill her for it.