Rune Assassin by Mac Flynn

The past comes back to bite Kate and Tegan in the butt as their blood-contract comes due.
Domini, the mysterious man who holds their contract, has commanded them to attend a prestigious magic tournament held by an old friend. Rumors have arisen of an assassin in the midst of the contestants and it’s Kate and Tegan’s job to root them out before the killer executes their plan.
The situation is complicated with the arrival of the contestants. The Keys have entered some of their finest into the tournament to show off their skills. The contest rules give Kate and Tegan amnesty for however long the tournament lasts, but the Keys are intent on bending those in order to capture them. That leaves the lovers in a bind as they have to navigate the ill intentions of the Keys with the murderous intentions of the unknown assassin.
The danger is high and the stakes even higher as time runs out for them. Will they find the culprit before the assassin strikes or will they be captured by their old foes to face an unimaginable punishment?