The legendary sword Excalibam- Thurston
Where do you get this sword
Edit: You need to do the following before you can craft Excalibam and expand to the Troll King's Lair:
1. Expand to the guy in his boxers
2. Complete his quests until he is dressed
3. Find a skunkupine to trigger the next quest (skunks like crops especially wheat)
4. When you have done that quest collect from Old Thomas' House to get the next quests
5. Do the next lot of quests
6. Collect from Thurston's a fort
7. Make sure you go into the Secrets of Mithril goal before sending your knights on the Dark Cave adventure
For details about the specific quests involved, see here.
Once you have defeated the Troll King you get a Ruby. The Troll Pit is also unlocked. You get coins when you collect from it.