Uki and the Ghostburrow by Kieran Larwood
 
'Why are you worried? You have us beside you! We've captured three spirits already. We've beaten Endwatchers, two clans, snakes, plagues and a whole ocean of disgusting maggots.'After capturing Charice, there is only one spirit left for Uki to find: Mortix, the most dangerous of all.

With his friends Jori, Cole and Kree, Uki heads to Eisenfell - the greatest city in Hulstland - only to find that Mortix has taken control of Emperor Ash and is plotting to conquer the whole Five Realms with her terrifying army. Uki must dodge the Endwatch, the Shrikes, Clan Septys and the guards and find a way to complete his quest before all is lost.