Before you can join the Dark Brotherhood, you have to attract their attention. To do this, you first have to kill an innocent NPC. Simply find a nonessential NPC and kill them. You’ll receive a message saying, “Your killing has been observed by forces unknown . . . ”. Find a bed to sleep in and Lucien Lachance, a Speaker of the Dark Brotherhood, will wake you. Lachance will instruct you to kill a man named Rufio at the Inn of Ill Omen, on the Green Road to the north of Bravil. This murder is necessary for admittance into the Dark Brotherhood. If you do not perform it, further progression down this line of quests is impossible.
Talk to Manheim Maulhand, the innkeeper, and he will tell you that Rufio is in the downstairs room through the hatch to the right of the front door. Rufio’s room is at the end of the hall. Kill him in whatever manner you choose and then sleep somewhere. I simply slept in Rufio’s own bed. Lachance will wake you again and instruct you to go to the city of Cheydinhal and find the abandoned house near the eastern wall. Enter the house through the basement and speak to Ocheeva, the Argonian. The achievement unlocks as soon as the initial conversation with Lucien is finished.