This dungeon can only be attempted at most three times a day, at either noon or around 10 PM server time, and it takes a team of six people. It takes place inside a large cathedral.
After an initial waiting period of roughly 60 seconds, 6 elemental statues (called Pillars in the instance) will appear spread out close to the walls of the cathedral. There is 1 per main element: Fire, Water, Wind, Earth, Light and Dark. Next to each statue, three Apostles (elementals) of the same element will appear and will start attacking the statue. Your team's initial goal is to kill off the apostles and save the statues before they are destroyed, 20 apostles per statue. The apostles will respawn at the normal monster respawn rate, up to a max of 3 per statue, and it takes quite a bit of threat generation to shift their targeting to you and off of the statues, so you'll have to act fast. The apostles give a very large XP bonus to the entire team per kill and the reward that an individual person gets scales by their level, making this instance one of the best sources of experience in the game and one of the best places to activate your Lionheart Scroll double-xp time.
Once a statue is saved, not only will you get an experience bonus but periodically an AOE energy blast of that statue's element will hit the floor, damaging everything in its area of effect (except another statue). The blast radius will appear on the floor and after a couple of seconds it will inflict the damage, so if you move out of the circle you will not be harmed. As more statues are saved, more energy blasts appear, making Priests very valuable allies to have.
If a statue is destroyed, much more energy blasts of that statue's element will appear for a while and you still have to kill the remainder of the 20 apostles for that element, except now they will openly target you and your teammates. It is usually best to tackle the statues in teams of 2.
One important note is on the use of your Soul attacks. You should definitely use these whenever all 3 apostles are attacking a statue at once. Periodically during the instance, your Soul Energy will be refreshed to max allowing you to shoot it again, and there is a good chance you'll level up at least once during this instance which will also refresh your Soul Energy to maximum.
With their AOE spells, Mages are the absolute kings of this instance as they are able to kill off the Apostles much faster than any other class, as long as they target the one in the middle of the group so that they hit all 3 on a statue. However, mage AOE spells really piss off Apostles and can quite easily pull all three of them onto you, so either partner with a Priest or have a lot of alchemy potions handy to counter this.
Once all six statues have been either saved or destroyed, a final boss will appear in the center of the cathedral. This part is pretty straight forward, just kill him, but keep in mind that the AOE energy blasts from all six statues will be pounding the area in a random scatter pattern, so you'll need a good priest or two or a lot of health potions to keep up. Once the boss is dead, the statue energy blasts will stop and you'll have a loot roll for a rare gem. That's it, you're done!