L'Emprise Du Sang (Amy Blankenship).

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Michael sometimes thinks that the blood of powerful immortals should not be mixed, even for two sisters who are consumed by passion. The marks of union are a symbol of belonging, but for Michael, the little taste of blood will precipitate his downfall. The blood of the Fallen is tempting to a Solar God and his swelling power that invades Michael plunges him into a deep addiction. In order to protect Aurora from himself, Michael sets out to hunt down the city's most powerful demons in order to satisfy his dark appetite. While the black blood pulses through his veins, Michael loses himself in the throes of this thirst and becomes as dangerous as the demons he pursues.

Bloodwield, by Amy Blankenship. If this is new to you, here's the plot summary I did rewriting in my own words: Michael sometimes thinks that the blood of powerful immortals should not mix, even in the fires of passion shared between identical twins. The mark of accouplement is a symbol of belonging but, for Michael, this hint of carnal bloodlust is all it takes to precipitate his fall. Blood lost to that of a fallen God, the Sun God's fallacious allure promises power to a Sun God who finds himself consumed by a deep addiction for it. In an effort to protect his love, Aurora, Michael sets about hunting down some of the city's most powerful demons to satisfy his dark grotesqueries. As black blood wells through his veins, he becomes lost along the furthermost path of his evanescent thirst, and as dangerous to everyone around him as the demons he hunts. Michael sometimes thinks that blood should not promiscuously mix, even between identical twin spirits caught in the fire of passion. The imprint of union is a mark of belonging, but for Michael, the hint of carnally-pleasing blood drops him over the edge. The blood robbed from the fallen God who is the Sun is alluring, its promise of power consuming the bright god who finds himself hopelessly addicted to it. To shield his beloved, Aurora, from himself, Michael sets out to hunt down the most powerful demons in the city, to fetter his voracious grotesqueries of desire. As opaque rivers of blood course through his veins and he fights to keep tumbling towards the darker depths of his undoing, he is only as dangerous as the beasts that he stalks.