Why are these people keeping a look out for people from this place (Secret History spoilers throughout)

Question

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?

Answer 1

I don't think anyone except Sanderson himself is likely to have a concrete answer to this question. Probably to actually figure this out is a matter of RAFO.

Nevertheless, I'll take a stab at it. Here is some speculations about why Sellish worldhoppers would care about Threnody.

One thing to note is that Threnody might not have always been as unimportant as it might seem. Someone has even guessed that Threnody was actually a shard odium splintered. While I can't say with much confidence if this is true... Note that this link does include a confirmation that Threnody is named in memorial of someone (which makes good sense considering the meaning of the word).

Also recall that the cosmere novels don't exactly take place all at the same time. During the time of Secret History, it isn't out of the question that Threnody has become a much more developed world then we see in Shadows for Silence in the Forest of Hell, and that Elantrians are at their pinnacle and not fallen like they were for most of Elantris. If the timelines coincide, it isn't impossible that Threnody and Sel were the two most cosmere aware worlds at the time, and saw each other as adversaries in some ways.

So, basically, my point is that while in Shadows for Silence in the Forest of Hell, Threnody seems like a much less interesting world than a major shardworld like Roshar or Scadrial... Some time before or after Shadows for Silence in the Forest of Hell, the world of Threnody might be more interesting. Even if the people of Threnody never actively use magic, they could have still compete with the Elantrians if they have a means of worldhopping (especially since Elantrians in particular have a much harder time using their magic off-world).

Answer 2

Have been listening to an audiobook of Arcanum Unbounded, which includes some notes by Khriss about Threnody. Apparently, Odium battled and mortally wounded Ambition in that system (though the splintering occurred later elsewhere). The effects of this battle have had large consequences for Threnody, including probably whatever "The Evil" is that has taken over the continent Shadows for Silence occurs on, and the process that creates the shades, which are shadows in the cognitive realm.

So, my semi-educated guess is what @Radhil said, that the shades are also dangerous, or even more dangerous in the Cognitive realm, them being cognitive shadows after all. My current mental model is to think of them as diseased versions of whatever Kelsier is in Secret History. Idk maybe this disease is contagious? That would line up with some of the vampire parallels of the shades in Shadows for Silence.

Total speculation: In Secret History, Kelsier notes that metal shows up brightly like souls in the cognitive realm. At one point, I think he even says they are the same or something similar. Perhaps the avoidance of fire lighting, blood shedding, and fast movement on Threnody have something to do with things/actions that show up more strongly in the cognitive realm on Threnody, as metal shows up strongly on Scadrial. Like I said, total speculation.

Another unrelated guess: I conjecture that the lack of magic on Threnody is also related to the after effects of this battle, as maybe the mechanics of investiture were damaged or affected in some way in this system.