The Devil’s Bargain. The Devil's Gentlemen, #51

Aurelius has survived centuries of darkness, supernatural wars, forbidden contracts, and enemies who believed they could own him. But the most dangerous bargain of his immortal existence is finally coming due. When an ancient entity known as the Collector demands payment for a centuries-old promise, Aurelius is given an impossible choice: surrender his supernatural power, sacrifice his memories of Marcus, or become the Collector's servant forever.
Aurelius refuses to sacrifice the man he loves. But the bargain has already begun to spread to Marcus. As Marcus starts losing precious memories, Aurelius and Marcus uncover a terrifying secret buried beneath Aurelius's mansion. Someone altered the original contract using Aurelius's blood-and the architect behind the deception may be Lucian Veyre, a powerful and obsessive figure from Aurelius's forgotten past.
Lucian wants Aurelius back. The Collector wants the promise fulfilled. And Marcus refuses to let either of them decide Aurelius's fate. Their search leads them into forbidden libraries, ancient supernatural domains, and a gothic world where promises can become weapons and love can become the ultimate act of defiance. To save Marcus, Aurelius may have to surrender everything that once made him powerful.
But Marcus discovers something even more dangerous: the original bargain contains a hidden clause that could change everything. When the truth is finally revealed, Aurelius must choose between immortality and freedom, power and vulnerability, eternity and love. Because some bargains are made from fear. Others are made from desperation. And the most powerful promises are the ones freely chosen.