The Liar's Key by Mark Lawrence (The Red Queen's War #2)

The Red Queen has set her players on the board...

Winter is keeping Prince Jalan Kendeth far from the longed-for luxuries of his southern palace. And although the North may be home to his companion, the warrior Snorri ver Snagason, he is just as eager to leave. For the Viking is ready to challenge all of Hell to bring his wife and children back into the living world. He has Loki’s key – now all he needs is to find the door.

As all wait for the ice to unlock its jaws, the Dead King plots to claim what was so nearly his – the key to the underworld -- so that his dead subjects can rise and rule