One Has To Die by MN Bennet

No longer haunted by the loss of Finn, Dorian’s finally moving forward with his life and finding happiness at Milo’s side. All he wants to do as his students move into their second year at the academy is prepare them for the industry and build a future with Milo.
Unfortunately, a stray piece of magic has sprung to life with plans of his own. This rogue manifestation has access to Dorian’s power and his memories, and he intends on using that knowledge to steal Dorian’s happy ending. Believing himself the superior half of Dorian, he’s done the unspeakable, harboring Finn and the devil that nearly destroyed the city. Now, he’s plotting to bring down the witch who failed to master his magics, failed to save Finn, and failed to embrace Milo while lost in grief.
The manifestation doesn’t care who pays the price of blood, so long as he claims Dorian’s body as his own. He’ll stop at nothing to achieve a perfect future with Finn and Milo.