Heart of Dragons by Aja James

Every lore of every clan, across time and space, dreamed of dragons. Did you ever wonder why?
Benjamin D’Angelo has officially replaced Ere the Black Dragon in the Jade Emperor’s mysterious quests. This time, Ere insists upon tagging along, for next to his Phoenix Mate Sorin, Ben is the greatest love of his life, and he just doesn’t trust anyone else to take care of his boy.
As always, their success is limited by seemingly arbitrary rules and an even more arbitrary sandglass of time. The only things they can count on is that: 1) each of their quests takes them to a time in the past, 2) they will witness (and perhaps even enable) True Love along the way, and 3) the Universal Balance hangs in…well, the balance.
Their sixth task is to retrieve the Stone Heart.
Ben and Ere return to medieval Britain some years after Ere’s first quest, reuniting with old friends in the process.
Meanwhile, Merlin the red dragon has escaped from the Celestial Realm to present day New York City. By luck or fate, he meets Annie, a fire witch, in his human form, drawn to her unique light. The present and past collide in the discovery and recovery of the Stone Heart, old wounds heal, and new Bonds are forged.
The end of the world may be just around the corner. Along the way, enemies may become lovers, reluctant friendships may form, and lots of hot, desperate, this-may-be-the-last-time-we-have-sex sex will definitely be had.
And who knows, you too may help earn the…