I am going crazy trying to finding a manga!

So there was a yaoi manga that I can't remember for the ever lasting life of me!
It was about the mc who was in a village, he had a boyfriend but his brother killed the mcs boyfriend!
The bf came back as some kind of merman monster thing. Wanting to drag mc into the afterlife.
But that's all I remember! If anyone knows what I'm talking about and can help me I appreciate it!