When you were speaking with Lucan D'Lere, he left you with a final clue: "Unfortunately, your skills are not as grand as I was led to believe. However, you can redeem yourself. When my expedition is complete here, infiltrate this throne. Seek and destroy the one that keeps me from the key I seek. He has not made his presence known, but I can sense him cowering in the shadows of my overpowering radiance. Perhaps he will let himself be known when I am gone."
The 'one' you must seek out is none other than Mayong Mistmoore. Although the quest text makes it seem like you must defeat him in Throne of New Tunaria, you will get credit if you kill him in any of the three places he appears: Castle Mistmoore, Mistmoore's Inner Sanctum, or Throne of New Tunaria. There is no quest for this; simply slay Mayong and speak with Sir Trodonis. You will be able to upgrade your Lucanic weapon into one of the following: