Rillian

Yeah, I think that's why she has drawing Urithuru, because something off about it in the CR.

I think that was because she has a strong enough presence in the cognitive realm, that her drawings come largely from there, so the spren appeared in their full CR form.

It's possible, but combined with the fact that it's in her personal notebook she keeps with her, and that she makes a habit of keeping herself from knowing things and splitting her personality...I think it was her.

Relevant WoB:

I don't think it's likely he would steal her pad without her noticing, and have the artistic ability to draw creepy horse deaths. Also, the text says that "she" drew them, so even though she doesn't remember doing it, the wording seems to tell us the did

I think it's a result of Urithuru and/or the Unmade thing interacting with Shallan's powers and split personalities

I don't think all listeners will end up being with Odium, but the fact that the Everstorm healed them, and then immediately a spren that Syl explicitly says is a voidspren. The spren can also sense the Everstorm coming, but not the highstorm. All in all, that's pretty strong evidence it's a voidspren

The 13 Royal Knights from Digimon Organization XIII from Kingdom Hearts The 13 Lords of Chaos from adventure quest