Cerberus

One, two, monsters are hunting you
Three, four, can't lock your doors
Five, six, send you to the River Styx
Seven, eight, better stay up late.
Nine, ten, you'll never rest again

For the last seventy years, spending my life as an assassin made sense. The goddesses of retribution and justice would give me the name of an immortal, and then I would annihilate them. They raised me to kill or die, and it was an easy choice.

I killed without mercy. I killed without thought. I kill as I'm told.

One cold afternoon, they sent me to retrieve an ancient artifact from an auction. Stealing the artifact would have been easier if a deceitful fox shifter hadn't sung me a song about my inevitable death. I couldn't escape the bounty placed on my head. For the first time, I disobeyed orders and ran.

By the time I realized my mistake, the artifact was gone, stolen by a hound I never thought I'd meet-Cerberus.

He shifts into a they, and they want me for reasons that have nothing to do with that artifact and everything to do with fate.
It doesn't matter how much I want or accept them, because I sold my soul centuries ago and I have nothing to give them aside from my heart, which will never be enough.

If love can't save me from my imminent death, I don't know what will.