Just recently, I've completed a game known as The Legend of Zelda: Parallel Worlds.

The game is a ROM hack of the popular Zelda 3: A link to the past for the SNES, the hack changes MANY, many aspects of the game, there are new graphics, harder enemies, new effects, new overworld, new music, new dungeons, new story and much more!

The story of the game is that there was a war for the triforce, and a parallel world was created as a result, there's 7 maidens who know clues to climbing the parallel tower where the Triforce resides and they seek to make their homeworld better by using the power, however when they get to the other world they get kidnapped by a king called Draegor who wants the power for himself.
The king Draegor used to be the King's assistant, but killed him and took over the throne, so a resistance is formed to take down Draegor, that's where the game starts off, with you and your friend trying to rescue the last surviving Maiden and to over-throw Draegor.

The story is a bit weird, obviously limited to what it can be about by the fact that it's a rom hack, but it works, what really matters is the gameplay.

The difficulty of this game can be especially cruel even to Zelda experts, you'll need to take advantage of MANY tricks you've learnt from the original game just to survive, and some just to progress!
The first dungeon itself requires you to go through half of it WITHOUT your sword! This wouldn't be so much of a problem if it weren't for the fact that you cannot save until you get it!
As if that wasn't enough, the first of the 3 main light-world dungeons has an insane mini-boss, it's the Armos knights! Just look at the screenshot below, you have to take these guys on with no arrows which means they take a long time to kill, you do it with the weakest sword, minimal health and a huge hole in the centre, it's not easy. (As a matter of fact, many dungeons take later bosses, give them a difficult surrounding and make them a mini-boss!)

Thankfully, if you play around a while before going to the first dungeon, you can actually beat this mini-boss quite easily, let me explain.

First off, you need to play around a bit and get some cash, if you look around you can find a mini-game to really rake in the Rupees.
After you get enough, you need to find the Zora scale so you can swim.
Next you need to go and find some mysterious arrows somewhere and memorise them.
Then you have to go an endless beach and make use of these directions to find an ancient pyramid.
After looking around a bit, you find a fairy fountain, you use your rupees to get 99 bombs and 99 arrows, you'll need them.
Next you goto the Misty forest and get a bottle, then you travel deep in there and through the lost woods, you have to figure out a combination that isn't anywhere in the game!
You then find a secret meadow, you get another bottle here.
After that you goto the village and get a bug catching net.
Next you goto get your bottles filled with fairies.
Then you goto the secret meadow AGAIN and enter a place called the halls of pain.
This is the cruel part, you have to use your bombs to hurt yourself, and propel yourself over holes, ouch.
If you're successful, you find an extremely useful treasure called the cane of Bryna.
Next you have to go look around for the Potion shop and buy 2 green potions.

...THEN, you finally go and fight the Armos Knights mini-boss, you use the cane of bryna to make yourself invulnerable, and run into them whilst the cane does a lot of damage one hit after another, that way you end up hitting 3-4 of them at a time without getting hurt, you use the magic potions to use the cane at least 3 times and if you're lucky you'll beat those bastards.

...Believe me, this long and winded process of fighting this boss like this, really pays off later in the game, the cane of bryna will be a critical item in the game.

Anyhow, the game is EXTREMELY challenging, there's even a second quest for the best players! Good luck beating this beast of a game if you get a chance to play it! (Because I ain't giving you a ROM to patch!)