I'm not sure if this is a book or comic book, but it's about a human city separated by a massive wall from a "monster" city (giant beasts, liches, goblins, witches, etc).
It's futuristic and fantasy at the same time, I think it's another planet.
Once a year, 5-6 teens are sent as ambassadors/emissaries/sacrifices, and I believe they never come back.
This year the city leaders have a plan to have the kids steal some orb of power that's on the monster side.
I think stealing it generates some huge cataclysm on the entire planet.