Devil Square (often called DS) is an small event (between level 50 and 160) where you can earn more exp, gold and drops than the normal world. When joining a Devil Square Event through the NPC Drul´jan (which can be found at Laurum Harbor) you are teleported to a small room. The goal is to survive as many waves of mobs as you can. The difficulty increases as you advance and you can kill other players at wave 5+. Devil Square is not only a great place to earn experience and gold but also to get unique skills (see Class Skill Books), and also corrupted jewels (see [Wiki] Upgrade System & Jewels)
There are specific level requirements for each DS (for example only Lvl 90-100). The difficulty of the DS is based on that required level. If you want to join a DS you have to wait for the 20 second long registration period, in that you can choose to join a DS by talking with Drul´jan and clicking on “REGISTER NOW”. DS events with the levels 50, 100 and 150 are announced in the #DSCHAN channel’s chat. (in game type: /join dschan)
The most common strategy to get as much exp as possible requires your team to stand still in the middle of the “arena” and spam your area skills (AOE SKILLS). In the best case you should also have one paladin or more in your party because of the extremely helpful FINAL PROTECTION skill, that helps you to survive longer as you advance.
Doing DS you can get some special titles (see [Wiki] Character Titles)