>>529746731
The monsters in that part of the dungeon are meant to be based on the townspeople. Dess is included in them in an "innocent" form.
ERAM is what Dess is now, The Knight. Same way Flowey isn't really Asriel, The Knight isn't really Dess.

>>529746924
Anon, the voice outright says it's HOLDING the Shadow Mantle. It's the person that took it from Seam before he could give it to us.
It's not the Mantle itself.