Patron of Mercy by Sam Burns

Lach has spent the last few thousand years counting only on himself. What he needed, he took. What he wanted, he won with charm. All except a god he turned his back on an age ago, when he had a different name and didn’t know what he was giving up.
Thanatos, god of merciful death, is one of the gentlest gods in the pantheon—easing the transition between life and death for billions of mortals. But he has faced eternity alone. After breaking his heart on the sharp words of a fisherman’s son, he hasn’t been able to connect with anyone.
Now, Lach is crashing back into Thanatos’s life, dragging him into an adventure that could save the world . . . or kill them both.