Fire and Earth. Elemental Union, #1

Benedict and everything he's been working towards are in danger. He hates the thought of having a bodyguard, but his son Rhea doesn't give him a choice in the matter. Benedict supposes he should be grateful that the man chosen to keep him safe is a friend of both Rhea and Rhea's soulmate, but things are awkward, especially when Benedict and Dakota have to fake being in a relationship. Dakota will do everything he has to in order to protect Benedict from Purity, the group that wants element wielders to stay separate.
Rhea is the soulmate of one of Dakota's best friends, which makes all of them family of a kind. Dakota doesn't mind faking being in love with Benedict, mostly because he suspects it would be too easy for that to become reality. When they're attacked and the elements they control twine together rather than fight, they realize they're mates. That won't stop Purity, though. If anything, it will make the cult even more dangerous, because that's exactly what they're against-different elements mixing together like they did before the war.
While this book can be read as book one in the Elemental Unions series, the short Elemental Union is a prelude to it. It's the story of how Rhea and Quillan met.