Gates of Wrath by Tamar Sloan

Saving those you love comes at a price.

Choices were made. Consequences were inevitable. And now, another Gate of Hell is the least of their problems.
The obsidian is on the brink of wreaking global death and destruction, and it’s Arielle it wants. The only way to save her is the use of dark magic to trap the obsidian’s evil, no matter the cost.
Meanwhile, as Reign learns there might be hope for the Keepers of the Grail, he also discovers the deep, dark truth about himself. Just as he thought he knew who he was, his foundations crumble.
To have a chance at victory, a fragile team is going to have to learn to trust. Another Innocent must be found and saved. Uncomfortable truths must be faced.
And yet, Wrath will be the greatest Sin they’ve faced yet.