For this quest, you will need the following:
-A Wailord. Capture a Wailmer and train it to Lv.40 to make it evolve, or
catch a Wailord on Route 129. They're very rare, so it's probably easier just
to raise Wailmer...
-A Relicanth. Search the area around Sootopolis City (underwater). They are
rare, and may take a while to find.
-TM28, Dig.
-HM2, Fly.
-HM3, Surf.
-HM4, Strength.
-Patience.
Teach Dig to any Pokemon . If you can't find one, look on Route 116 for a
Nincada. They can learn Dig. Trapinch, found in the desert, can also learn Dig.
Teach Wailord or Relicanth Surf and/or Dive. Make your way to Pacifidlog Town
one way or another. If you've never been there, Fly to Mossdeep City and Surf
as far south as you can go, then surf west until you get there. Surf west from
the town until you reach the currents.
Surf all the way down and stay down. Stay as low as you can until you're on an
island that is partly on Route 133 and Route 134. Get in the lower left corner
of that small island. Step up two spaces and Surf left. Dive and enter the
door.
Surf down the cavern until you reach a rock with Braille on it. Resurface to
enter the Sealed Chamber.
Here's a small picture in case you don't get it.
******************************************************************************
Message from sign in the Underwater Cave on Route 134
"GO UP HERE."
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First room in Sealed Chamber messages
|############|
|##DIG HERE.#|             Braille translations.
|            |       Use Dig where it says "DIG HERE."
| ABC DEF  . |             to open the pathway.
| GHI JKL  , |
| MNO PQRS   |
| TUV WXYZ   |
|     __     |
|    |  |    |
--------------
^ Water where you surfaced
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Go to the back of the room and stand in the middle of the Braille. Use Dig to
reveal a path. Enter the next room and head to the northern end of it. Put
Relicanth in the first spot in your roster, and Wailord in the last. Press A
on the Braille, and the doors to the Regis will open!
Braille messages in Sealed Chamber, second room
###############################################################################
##################FIRST COMES RELICANTH.  LAST COMES WAILORD.##################
IN THIS CAVE WE HAVE LIVED.              WE OWE ALL TO THE POKEMON.
BUT, WE SEALED THE POKEMON AWAY.         WE FEARED IT.
THOSE WITH COURAGE, THOSE WITH HOPE.     OPEN A DOOR. AN ETERNAL POKEMON WAITS.
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Use Dig to leave the cave. Surf back out of the chamber and resurface. Welcome
back to reality. Time to catch us some legendaries!