You get here by taking to Ange Rassa. You can get to her through a quest on Varanas Plaza. Once inside, you talk to the man standing there and hand over 30 Phirius Token Coins. Go through the doors, and you end up in room 1. If you're low level, use hide to avoid the knights, and snipe on the dogs.
Room 1: go to the brazier and acquire the flame, avoiding the enemy walking around. Once you have the flame, go to the "cave ghost" (basically a force field) and touch it, which will make it disappear, giving you an opening to room 2.
Room 2: this room will have an unlit brazier and a dog. Go to either lit brazier (room 1 or room 2) acquire the flame and light the unlit brazier. In the wall through which you came into this room are other openings with cave ghosts, either to the left or to the right. Go tot he farthest one and touch it. This gives you an opening to room 3.
Room 3: go to the brazier (or the one in room 2) and acquire the flame. Go to the other exit from room 3, which leads o a room very similar to room 2, which is room 4.
Room 4: this room will have an unlit brazier and a dog. Go to either lit brazier (room 1 or room 2) acquire the flame and light the unlit brazier. In the wall opposite to the one you came in through will be one opening, open this one and head to Room 5.
Room 5: This is a small circular room with an even smaller room in the middle. There is also an unlit brazier. Light the unlit brazier and the dogs and knights and everything will soon go away. A door to the inside smaller room will open. Go through and open one of the chests. This will give you the 30 Phirius Shells you so desire.
The chests on the left and right also contain Phirius Token Coins, which can be useful, and guild donations. The chests at the back contain useless runes. The chests to the front are only there if you completed the treasures fast enough, and give you pieces to build one of two different types of awesome 30 day mounts.