In The Elder Scrolls V: Skyrim for the PC, PS3, and Xbox 360, just like in previous games you can become a legendary assassin by joining The Dark Brotherhood.
To join The Dark Brotherhood, travel to Windhelm and look for the Aretino Residence. Talk to the boy there named Aventus Aretino and do the “Innocence Lost” quest. A little while after you complete the quest, a courier will deliver a note to you. After you receive the note, sleep in a bed and you will be contacted by The Dark Brotherhood in the middle of the night to join their secret guild.