The Infernal Underground by Megan Linski

Love takes a scandalous vow.

Ava-Marie
Things at the Institute look darker than ever. I’ve lost all my friends, my hope, and the person I love.
To make matters worse, the prison is planning something nefarious. Experiments are being performed on inmates at the Institute, to satisfy the Warden’s craving to find a demigod.
I’ve sworn to uncover the Warden’s plans, and go underground to learn just what he’s plotting. Unfortunately, I can’t find answers without putting my soul on the line.
And the consequences are going to propel me straight to the afterlife.

Charlie
The Warden promises the Institute is safe, but I know better. He’s hunting people down– people like me. To protect us all, we must start a revolution.
I must step up and become the leader everyone is searching for. I’ve decided to start teaching magic in secret to a group of prisoners I trust. When the time comes, we’ll be ready to face our enemies.
Students are disappearing from the prison, and we don’t know where they’ve gone. Now, I’m next in line to vanish.
But there’s a way around the law. To remain where I’m needed most, Ava and I will have to make a promise that will change our entire lives.
Sharing a bed with the enemy is one thing. Marrying her is another.