Sir Thursday.

In the fourth installment of Garth Nix's 'The Keys to the Kingdom' series, Arthur Penhaligon finds himself trapped in the magical House, drafted into an army, and battling a powerful foe, Sir Thursday, for control of the fourth Key. Meanwhile, on Earth, a magical doppelganger of Arthur unleashes a fungal infection, threatening his friends and home. This young adult fantasy explores complex themes of power, identity, and the struggle between good and evil, with Arthur's natural compassion tested against the harsh realities of the House. It's an action-packed adventure praised for its originality and intricate world-building, though some readers note a slower pace than previous books. The narrative shifts between Arthur's journey in the House and his friend Leaf's efforts on Earth, offering multiple perspectives on the unfolding crisis.