Mirror World

The Mirror World is a special place unlocked using the Full Mirror Piece, obtained from The Book Repair Shop using 500 Mirror Shards from around Dream World. It is host to extremely useful pieces of equipment.

Location
Southwest of Whispy Woods lies the Ruin Gate. At the top of the grey structure is a tile that will open a passage at the front of the building, leading into the Gate Entrance. Proceed through the Gate Entrance to access the Mirror Chamber, where you can use the Full Mirror Piece to access the Mirror World hub.

Hub Area
The hub of Mirror World is the gateway to all 4 sects: Grass Plains, Ocean Valley, Star Space, and Lava Hills. Each one is host to a piece of mirror-themed gear, with Star Space having 2 of them. The red gems at the center will take you back to the Mirror Chamber.

Grass Plains
This room is host to multiple rock pillars, one of which is hiding the mirror gem. Walk behind the pillar on the top right, and walk down once you're centered. Pressing enter should pick up the mirror gem hidden there. Any of the star blocks surrounding the room will warp you back to The Hang-out, so tread carefully.

Ocean Valley
Walk directly down into Ocean Valley Town, the mirror pants are on the right side in plain view. You can continue heading south of here to eventually reach Pie Lake, part of Pie Town. This is the only part of the Mirror World that's non-isolated, flowing directly into Pie Town. Watch out, though, because you can't walk back into the mirror world once you leave the Mirror World Ship.

The un-forgetable flame path
This part of the mirror world is full of instant-death tiles, tread carefully. In case it wasn't obvious, most of them are flames/lava. Head north into The flaming town that fell, then east into The falling path of flames. Head down the ladder at the center of the map to reach The Falling Flame's final resting place. Here, you can obtain the Mirror Shield. There is no warp out of this section of the Mirror World, though walking into an instant-death tile will get you out.

Mirror Space
This place is host to 3 houses, though they serve no practical purpose. Head north to reach the field, where you can claim the Mirror Slasher and Mirror Armor.