Pokémon Red and Blue/Unknown Dungeon
After the credits roll, when you continue your game you will find yourself in your old room in Pallet Town. All your Pokémon are still there and still just as powerful. From here on out, you can rebattle the Elite Four whenever you want. (If you’ve been defeating every enemy Trainer on every map, this is now your only way of acquiring funds.) But there is still one last thing you have yet to do…
Fly to Cerulean City. Stop at the Pokémon Center and take out that Master Ball you’ve been saving; you’ll need it. Go north to Route 24, and surf southwest, following the path of the bridge. You will reach a small area with a cave. If you were to access this area any time before becoming the Champion, you would find someone here standing in your way. But now you are the Champion, and you now have authorization to enter the most complex cave in the game.
The Unknown Dungeon is a big scary area, filled with high-level wild Pokémon. If you are playing Pokémon Yellow, then no guidance is necessary for this dungeon; it is a simple linear path, back and forth and up and down. If you are playing Pokémon Red or Blue, read on.
Go up the stairs to the first ladder, leading up. If you want a PP Up, then go up it and go through the maze; you will emerge at a separate ladder not far from here. Otherwise, ignore it and start Surfing. Ignore the first three ladders you may find. The first one is the out-point of the path to the PP Up; while the second and third ones are the beginning and end of another side path that contains a Full Restore. If you search the area you’ll find a white-colored rock near the third ladder; this contains a hidden Rare Candy.
The ladder you want to take is at the very end of the path. Up on the second floor, just follow the maze. No items can be found in any of these dead ends. Once you go back down to the first floor, you will be right next to a ladder going down into the basement. A Max Revive can be found to the east of the ladder. Otherwise, this is then a straight shot to Mewtwo. You will pass an Ultra Ball on the way, but you won’t be needing it. You can find a second Ultra Ball hidden in the white rock at the northeast corner of the map.
Boss — Mewtwo
- Level 70 Mewtwo
Throw your Master Ball and the battle is over.
If you used your Master Ball on a different Pokémon, you will find Mewtwo an extremely powerful Pokémon. It makes catching Zapdos seem like a picnic. It has Recover, so this will be a very long battle. The easiest tactic is to try to freeze him by using Articuno’s Blizzard and Ice Beam repeatedly. Once frozen, he will not thaw unless a Fire-type move is used on him, or you use the Haze move.