In the mausoleum, past the portal, there are more of those invisible platforms. There are two areas accessible by them. The right one has a chest that you need the key of the swarm for. The left one spawns three Blind Thralls that are invincible and a chest saying A scent is the key.
Does anyone know how to open the chest?
I think you need some item that will make the thrall stop attacking you and open the chest.
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In the same area there is a chest (forgot where it was) but if you stand in front of it for long enough your screen will start to have a green tinge to it, then you get a timed buff that will let you open that chest.