To join the Dark Brotherhood in The Elder Scrolls IV: Oblivion, you must first kill an innocent NPC who is not essential to any quests or the main storyline. This initial murder signals the Dark Brotherhood to you, indicated by the message "Your killing has been observed by forces unknown…" After this, you need to sleep in a safe location, such as an inn, where you will be visited in a dream by Lucien Lachance, a Speaker of the Dark Brotherhood. He will offer you membership and give you a first contract to prove yourself

. The typical first target to kill for joining is Rufio, who resides in the basement of the Inn of Ill Omen north of Bravil. After killing Rufio, you rest again and then proceed to an abandoned house in Cheydinhal, where you use the passphrase "Sanguine, my Brother" to enter the Dark Brotherhood sanctuary and officially join the guild

. Once a member, you will receive contracts to kill various targets, such as Captain Gaston Tussaud, and progress through a series of quests involving intrigue and assassination within the Brotherhood, culminating in becoming the Listener, the leader of the Dark Brotherhood in Oblivion

. In summary, the key steps to join and whom to kill are:

• Kill an innocent, non-essential NPC (e.g., Rufio at the Inn of Ill Omen)
• Sleep in a safe place to be contacted by Lucien Lachance
• Complete the initiation contract by killing Rufio
• Enter the sanctuary in Cheydinhal using the secret passphrase
• Begin taking contracts as a full member of the Dark Brotherhood