Roughly midway through the ebook Mistborn: Secret History Kelsier discovers a keep/castle of sorts. We can deduce from the decriptions of both the "elders" giving orders in this place and the phrasing they use (references to Devotion throughout) that they are from the world of Elantris and The Emperor's Soul.
It is noted that there is
Nobody from Threnody within a days march
Why are Elantrians keeping a watch for people of the minor shardworld Threnody (setting of the Shadows for Silence in the Forests of Hell shortstory)?
What we know so far about Threnody people is that there is next to no magic on the world as it manifests in a notably passive manner eg. Shadows. The are also governed by "simple rules", as described "no blood in the night, no fire in the night" etc.
Why would Elantrians be afraid of people from Threnody?