Upon returning to the Sea Scholar with the Magnifying Glass, he will tell you the following:
Deliver the Magnifying Glass
Thank you for retreiving the magnifying glass! I have finished deciphering the text. Are you ready?
It appears that this ancient treasure spoken of was swallowed up by a creature conjured by the ancient ones. They brought forth such a beast by using a variety of rare herbs and potions to make a brew that would lure it here, but weaken the one who drinks it.
While you were gone I took the liberty of finding these ingredients and brewing the potion. Since you've done what I've asked, I will give you this potion. Be wary though, this foe will likely be difficult and the potion will likely damage you upon drinking it.
The Potion of Trials may only be used at night. When you drink it, the message 'The Cursed Queen is near...' will appear and a few seconds will pass before you lose 30 health points and the Cursed Queen appears.
Be sure to have at least 31 health points before drinking the potion, or it will kill you.
Fighting the Cursed Queen is similar to the Spider Queen, in that she deals a large amount of poison damage on top of every one of her attacks. As such, it is a good idea to keep Bandages and White Flower Potions on hand in order to counter her poison.
When she has been defeated, loot her corpse to finally get the Treasure Hunter's Scimitar!