A power struggle exists within the guild between rogues "D" and "K." This quest is the culmination of "K"s quest line, he asks for the warden's help betraying "D" to the guards.
First all of "K's" friends must be given a signal. Gorim, on the south side of the market square, and Sister Theohild, outside the chantry, in the Denerim market, and the tranquil proprietor of the Wonders of Thedas. Talk to each and given them "K's" wink and nod and they will return the signal as expected. After all the signals have been sent, check in with "K's" Denerim guard contact. He's near the gate in the north west corner of the market. He will tell you to watch your back while traveling in Denerim. While traveling in Denerin you will have a random encounter with "D's" men, kill the Lieutenant to reveal the location of "D's" hideout. When you have "D's" location, simply go there and kill him.
When you return to the bartender, you will recieve 10g and a small amount of exp.