Time of the Druid by Ivy Hollins

A time-traveling assassin. A powerful druid. A decision that could change all of time… Norah is done with working as an assassin, done with having blood on her hands. But she can’t win her freedom until she carries out one final job - and this one will be the hardest thing she’s ever done. Matthew has turned his back on his father’s dark magic, but he still has secrets - dangerous secrets. When Norah stumbles into his life in Iron Age Scotland, he knows that she could ruin everything. But he still can’t stay away from her… Norah and Matthew are forced to make the hardest decisions they’ve ever faced. How can love win against Professor Edmondson and the whole weight of time?