Into Battle by Garth Nix
Tal must go to Aenir to use the Violet Keystone to free the guardian, Lokar and then free his father. But when he gets there, he realizes that he must get to the Empress's island, which is across a lake of ash with terrifying creature living in it.
Milla has been cast out of her clan, and then welcomed into a new clan as the War Chief. She is in charge of leading the attack on the castle to prevent the shadows from invading the Dark World. Inside the castle, they team up with the Underfolk and prepare to attack while the Chosen are away in Aenir. But someone alerts the Chosen and they come back sooner than they should. What will be the result of the battle? Will Tal reunite his family in time?