Troubletwisters4 books in series
Troubletwisters: Book 1 Publisher's Summary
A spectacular new middle-grade fantasy series from NYT best-selling authors Garth Nix and Sean Williams, in which two twins find that they must act as wards against a threatening evil. The Evil has been trying to break into our dimension and dominate the earth for centuries. Unbeknownst to most of us, there are Wardens all over the globe, who protect humanity from the Evil that asserts itself at the Portals, which are the only places through which the Evil may pass into our world.