Mallmoc's Castle - Number 2 in series

Jack's fought with giants, escaped the clutches of an evil warlock and rescued a damsel in distress, but he's still no closer to finding his way back home. Instead, Jack's caught up in Spiggot's crazy quest to defeat the dreaded Thrum, underworld faeriefolk who are destroying Liofwende's magical forests. If the quest is successful, the King of Northumberland just might help Jack return to the twenty-first century.
But if they fail, Spiggot and Jack will be at the mercy of the evil wizard Mallmoc - and that's a fate far, far worse than being stranded in a faerie realm forever. The Knights of Liofwende concludes with: 3. BOGGART AND FEN (July 2004)