You'll find yourself before a deep chasm. Your goal here is to jump around using the fixed platforms and the rotating discs. The Athena blockers (#1) will try to knock you into the chasm, but they won't move, and so you can kill them easily but slowly with ranged attacks. For a quicker way to deal with the blockers, you can walk around the edge of the chasm, stop right next to the platforms where the blockers are standing, and attack them from there. If your melee weapon has a long enough range, you should be able to hit them without them attacking you back. Even better, if you're proficient with heavy weapons, you can use a sword of stone or a giant stone sledge. Both are "pro vs. stone" and both are dropped by Athena enemies. Each blocker is worth about 400,000 experience.
If you end up in the chasm, you'll find yourself in the shadow realm, which looks remarkably like a maze of purple islands. To get back to the Shadow Ruins hub, you'll need to take the eastern gate, then the eastern gate, and then the northern gate.
To activate the shadow gate to the north (Exit B), you'll have to pick up the shadow crystal (#2) and place it in the altar of shadows (#3).
You'll find yourself in a room with three Athena guardians (#4). The guardians will start out inactive, so attacking them won't do anything. Your goal here is to go through the shadow gates to the west (#5), east (#6), and north (#7), and pick up shadow stones from the chests in the "purple island" mazes beyond them. Each time you pick up a shadow stone, you can use it on an Athena guardian in the main room to awaken it and activate another shadow gate. After you've awakened all three Athena guardians, the shadow gate to the south (Exit C) will activate, and you'll be able to enter the next section.
Here are the routes to take beyond each of the three shadow gates:
Door #5: East, east, west, loot the chest on the western side, south, east, eastNote: In MMXII, the Shadow Stones from the islands are orange, purple and red to differentiate them from the regular blue stones. Don't pick up these non-blue stones from the bodies of the Athena guardians, or you'll be stuck with them for the rest of the game.
Rune Glyph Section
This section is pretty straightforward. The rune glyph (#8) is at the far end of the room. After activating it, you can go through the shadow gate (Exit D) and return to the Shadow Ruins hub.
1 - Athena Blockers
2 - Shadow Crystal
3 - Altar of Shadows
4 - Athena Guardian
5 - West Shadow Gate
6 - East Shadow Gate
7 - North Shadow Gate
8 - Rune Glyph
Press the SHIFT key to activate (light up) the rune glyph.