Venture around the Wandering Mausoleums and you are bound to see Mausoleum Soldiers walking about. They guard the Walking Mausoleum against intruders like yourself who are trying to break in. As you journey around the Lands Between, you can find a summon that lets you have your own Mausoleum Soldiers to aid you in battle. Here is where you can find the Mausoleum Soldier Ashes in Elden Ring.
To obtain these spirit ashes, you first need to make your way to Deeproot Depths. This is an area you will be able to find after you have progressed pretty far into the game. There are two ways you can get here. First, you can ride the coffin up the waterfall after the Valiant Gargoyle boss fight. In the boss arena, interact with the coffin to get a cutscene where it takes you to Deeproot Depths. Second, you can go down into the sewers of Leyndell, Capital City. From there, make your way to the boss room of Mohg, The Omen. After the boss fight, go through the hidden door in the altar and fall down the giant hole. Hit the illusory wall at the bottom. This will open the door to Deeproot Depths.
Once you are in Deeproot Depths, make your way to the northwest to find the Nameless Eternal City Site of Grace. From the Site of Grace, head northeast until you find the Walking Mausoleum. Continue past the mausoleum to the cliff in the far north. Resting on the edge of the cliff will be the Mausoleum Soldier Ashes. Just make sure to watch out for the soldiers shooting great arrows along the way.
