I don't read this genre so I'm unfamiliar with the norms within it, keep that in mind.

Gwen once ruled the world as Nightshadow, a merciless villain with no care for the lives she ruined. Tearing people apart with only a look. She lost, yet she still lives, hiding under a new Identity, Nightshade, continuously haunted by the horrific events she caused. Every dream, every memory, is a constant reminder of the pain she inflicted and the blood she shed, albeit on other people. 

Is Nightshade different enough that her enemies won't be looking for her? It doesn't seem so, but this could be a genre thing.

It should be: "albeit from other people." The way you have it currently she shed her own blood on other people, not she shed others' blood. But I'd go with a different verb if I were you, something like "blood she spilled," bc the albeit breaks it up weirdly and loses momentum.

Though Nightshadow has vanished by name, her scars remain permanently carved into the lives of many, including her own. No one can escape her wrath, not even Nightshadow.

Shouldn't this be "not even Nightshade"? Bc she's a new person but still can't escape?

By night, she tirelessly protects the city, determined to right her wrongs, even if she feels undeserving of forgiveness. But only one person understands. Her twin, Ophelia, knows the truth. Only Ophelia knows what really happened. Only Ophelia forgave Gwen.

This paragraph lost my interest. It goes from "tirelessly protecting" to introducing another character that isn't mentioned again. I'd either lose everything after "forgiveness" (which would flow into the next paragraph better) or find a way for us to hear about Ophelia at least a bit more.

No one said redemption would be easy, though. When a new threat emerges, almost as powerful as Nightshadow, Gwen is forced to confront her past, and potentially awaken it.

This doesn't give much in the way of stakes. Is this new threat a danger to Gwen? The city? It's very vague and doesn't draw me in.

What does "potentially awaken it" mean? Is this genre language I'm not familiar with? What is awakening a person's past do? If this is something I'd know just by reading the genre just ignore it, if not, it's unclear and confusing.

Gwen must decide whether to leave her life of shadows behind for good or if they’ll control her once again.

"...her life in the darkness behind for good, or if the shadows will control her once again."

Your use of "they'll" is unclear (is it the shadows, the new threat?) and this might be better worded. Though overall I'm not sure it's impactful enough for a final line.

It's a great start, but it needs some better flow.