Cicero Puer gives this quest after you complete the Murkwater Uniform quest (see related quests).
Cicero will send you to kill the Crazy Peeper in the The Serpent Sewer. He spawns in the southwest corner of the zone. Clear the corner if he is not up and he will spawn. He is level 21 in a group of mobs.
After killing Crazy Peepers you go back to Cicero and he tells you that he knows someone who knows where the entrance to The Serpent's Lair is. He asks you to deliver a message to Tula Mawr in The Serpent Sewers and that she will tell you where it is. Tula can be found at -113, -0.2, +6.61.
When you deliver the note, the merchant tells you the message was actually a love letter. You go back to talk to Cicero, and he breaks down and tells you where the entrance to The Serpent's Lair is, and the quest finishes.
The Serpent's Lair is a instanced dungeon that requires 3-6 players to enter. You can enter it by clicking on grate on the east wall halfway between the drudge globule room and the northwest crazed menders room.