After reaching Riverwood, speak to the bartender inside the inn. Ask him if he has heard any rumors. He will eventually tell you about a boy that wants to summon the Dark Brotherhood. Follow the subquest to Windhelm. When entering Windhelm, make a sharp right and travel up the path until you encounter a Dark Elf and child talking. Enter the door to their left and find the child inside. He will ask you to kill Grinelda The Kind in Riften. Go to Riften, enter the orphanage, and murder Grinelda. Fast Travel back to Windhelm and talk to the child again to get your reward. Next, go outside of the city and wait for twenty four hours. A courier will approach and give you a note with a black handprint on it, with the words "We Know". Go to the nearest inn, rent a room, and sleep for twenty four hours. When you awake you will have been kidnapped by one of the Dark Brotherhood. They will tell you that if you kill one of the other three people in the room they will release you. Pick a person and kill them. Speak to the Dark Brotherhood again and they will tell you to meet them at their lair in the pine forest in the far southwest corner of Skyrim. Fast Travel to the western watchtower outside of Whiterun then travel west towards the marker. When you get to the door, answer the question asked with "Silence, My Brother" to enter the sanctuary. Speak to the first person you find inside the lair, and you are now part of the Dark Brotherhood.