First talk to Dark Recruiter Balin in Prontera, chose to create and enter Society Raid.
You will end up infront of Dark Raid Leader Sylvanas, and she will talk about how the Order of the Holy Circle are raiding their headquarters.
Then the Order of the Holy Circle ambushes you and you will have to kill them all.
Once you have killed 39 of the Holy Orders acolytes, try to enter the portal that has now spawned.
Constantine will appear and tells you to get away from the portal and that he will exterminate you by the Order of the Holy Circle.
Kill him off, after he is dead it will tell you the Holy Circle members have all been eliminated outside and to proceed inside.
Order Guard Gabriel and Pharell will be waiting for you inside and will try to kill you.
After you kill them you have to search every room and kill 15 Acolyte Guardians.
After you kill 15 of them it will tell you to use the elevator to get to James Laboratory.
Once you get up there you will meet High Priest Julius and his guards standing against Sylvanas.
Julius guards will kill off Sylvanas guards, and then Sylvanas teleports Julius away and warps away herself, meanwhile telling him she will take her time dismantling him.
Now walk to your right and you will encounter Decimus, who will be attacked by Sylvanas, all you have to do is avoid the spell he's casting and if you walk too far away from him he will start healing up so avoid doing that.
Sylvanas will cast Soul Shatter on him to damage him enough for you to kill him, while you avoid his Storm Gust.
When he is low enough on HP Sylvanas tells you to finish him off so do that, once he is dead you are done.
Congratulations, you have finished Society Raid!