Searching more about Book 6, and I was able to find the cover for the next book in the series, Escape From Lucien.

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(Link in image.)

Unfortunately, all that I've found of it is a thumbnail, so you can't see everything clearly. It seems to definitely depict Vigo, Emily, and Trellis. There's what appears to be a Colossus mech overshadowing them. And in the very back of it all, there's a sinister-looking face. It bears some resemblance to Trellis, but considering it seems Trellis is already on the cover alongside Emily and Vigo, this is likely someone else. A younger Elf King maybe? A new character?

What are your thoughts?