Shade’s Children is a young adult dystopia that I found intriguing. Quick notes:
The story’s main narrative line depicts an earth invaded by humans from the future who decide to turn the planet into a vast war game. All the adults were vaporized, and children left to become raw materials for the soldiers and other game pieces created by the Overlords.
Some of the children who escape form a resistance led by a ghost shell of a scientist. This shell goes a bit crazy but redeems itself in the end.
The creatures constructed are pretty terrifying, ranging from Myrmidons (soldiers) to ferrets (humans turned into blood-drinking weasels that have a free run of the planet at night).
All of the game pieces are reconstructed from the bodies of children. We don’t find out if the Overlords are not allowed to use technology because of the rules of their game or something else.