Once you have reached level 71 you are able to enter a dungeon in Kaslow and do a quest.
You can find the entrance at a NCP named Setmund, x=252, y=304. Near Mount Dealer.
After you have entered, you will see two NPC with quests.
Paraka (on the left) holds the quests for casting oriented classes, like Mystic, Prophet, Archmage and Demonologist.
Jeremy (on the right) holds the quests for physical fighting oriented classes, like Warlord, Templar, Darkstalker and Sharpshooter.
You can chose any quest you want to, but the target you will be fighting is going to drop a Power Stone of its class e.g. fighting a Warlord cause it to drop a Berserker/Warlord/Deathknight Power Stone.
Monsters might also drop Treasure instead of a Power Stone.
Once you take the quest you will gain a curse that will last 22 hours until you can take a quest again.
Hand in the quest to the same NPC and you will instantly enter battle with your chosen target.
Once you have killed it, the target will drop an item which you can take. Then you are done with the dungeon.
You can enter the dungeon again but there is no quest for you to take.
If you enter battle and leave the target will still be there for 3 minutes.
If you're having trouble you can ask a friend to help, party up and kill bosses together.
If you don't need the power stones you can always try other classes for fun.
Before handing in the quest, its a good idea to use your buffs.