Instructions
Go to the Mushroom Castle, use a Poison Spore to enter and climb up the vine to reach the throne room. Here, you have to clear the castle of different colored Yetis and the boss monster, King Pepe. When King Pepe is defeated, he drops weapons and armor, with a great chance of dropping a spear.
Take the mission from the Captain of the Guard for King Pepe's scroll. For this mission, you have to kill 200 Mushrooms and 400 Helmet Pepe, which takes a few hours in game time. Return to the Captain to claim King Pepe's Scroll.
Click the scroll in your inventory and drag it over to the weapon dropped by King Pepe. These are always obvious, because they're called "King Pepe's Spear" or "King Pepe's Sword," as an example. This scroll can only be used on these types of weapons.