Unfortunately, the map doesn't become much of any help ... You'll have to work your way through the Subterranean Shunning Grounds. This is a long and tedious process with many winding tunnels ...

You can find these spirit ashes at the location indicated on the map image below. These ashes can be looted from a ledge in the Subterranean Shunning-Grounds underneath Leyndell, Royal Capital.