Eldritch

Personally, even if it is a shard, I don't think that Trell is going to be Odium. For starters, pretty sure there's a WoB that Odium is supposed to be quite scared of Harmony, with the whole holding two shards thing. Also I agree with the idea that if it were another shard, it's origins would be more obvious to Harmony, whereas his uncertainty would suggest it's something else. Has there ever been WoB about any other individuals with a level of power similar to that of a shard? Other than the sixteen obviously.

Maybe this isn't really relevant, but to say they were all (somewhat obviously) Elantrians, none of them used any form of AonDor insofar as drawing symbols at all, even when they thought "ruin" was after them. Now of course it might just be they didn't think it would be any use against a shard itself, but you'd think at least one of them would try it if they were capable. So for one reason or another I'd say they they weren't able to use them.