Wonder Boy in Monster Land

Don't fight the secret boss while descending the Round 8 cave (don't knock on his door,) and you will receive the Sword of Legends from the Giant Kong in the Round 10 underwater cave. Make sure that you save before fighting him, because there's a chance that he may not drop it the first time you beat him. If this happens, then reload your save until you obtain the sword. To defeat him, just do what you did with the boss in Round 5. Keep going towards him and attack. As soon as he jumps over you, then turn around and repeat.

You can also obtain equipment from a fallen enemy on the Knights outside the Dragon's Castle in Round 11. I got two item drops here.