Marius don Hellespont, book 1: The Corpse-Rat King
Lee Battersby2012

Marius dos Hellespont and his apprentice, Gerd, are professional looters of battlefields. When they stumble upon the corpse of the King of Scorby and Gerd is killed, Marius is mistaken for the monarch by one of the dead soldiers, is transported down to the Kingdom of the Dead. The dead need a King--the King is God's representative, and someone needs to remind God where they are.

Marius is banished to the surface with one message: if he wants to recover his life he must find the dead a King. Which he fully intends to do. Just as soon as he stops running.