Definitely Dead by Arden Steele

As a royal Guardian, Tyr Bergstrom is built for battle, not stillness. Not whatever this is. He followed his prince into the depths of the Underworld out of loyalty, but with no foes to vanquish and no lives to safeguard, he’s left questioning why he stays. It’s dark, cold, and each day blends into the next with monotonous sameness.
Until a spark of sunshine arrives in the Village of Lost Souls.
Sunne Tanaka died. Not ideal, but he’s had worse Mondays. While the afterlife isn’t anything like he expected—frankly, it’s a total scam—from the moment he meets Tyr, he can’t help but feel that he’s exactly where he belongs. He hasn’t decided if the shifter is his destiny or a beautiful disaster waiting to happen, but he knows one thing for certain. He’s not leaving the village without him.
But Tyr knows better than anyone that some lines aren’t meant to be crossed. He doesn’t know what waits beyond the dark waters of the River Acheron, but it’s the only condition of his stay in the Underworld. A rule he has never questioned…until another soul puts Sunne’s very existence at risk.

Now, he’s not just questioning.
Now, he’s prepared to take on the living, the dead, and Hades himself to bring his mate back and give them both a chance at a happily ever afterlife.