To join the Dark Brotherhood in The Elder Scrolls IV: Oblivion, you must follow these steps:

1. Kill an Innocent Person
   The only way to attract the Dark Brotherhood's attention is by murdering an innocent NPC. This cannot be a hostile character like bandits or quest- essential NPCs (marked with a crown icon). The victim should be a non- essential, innocent person who is not involved in quests or important to the story. A common tactic is to kill a lone beggar or an isolated NPC at night to avoid witnesses

2. Sleep in a Safe Place
   After committing the murder, go to sleep in a safe location such as an inn or a bed you own. Upon waking, you will be visited by Lucien Lachance, a Speaker of the Dark Brotherhood, who will acknowledge your deed and offer you a chance to join

3. Complete the Initiation Task
   Lucien will assign you a task called "A Knife in the Dark," which involves assassinating an old man named Rufio. Rufio can be found in the basement of the Inn of Ill Omen, located south of the Imperial City. He is defenseless and asleep at night, making the kill straightforward

4. Officially Join the Dark Brotherhood
   After killing Rufio and sleeping again, Lucien will officially induct you into the Dark Brotherhood. You will then be directed to the Dark Brotherhood Sanctuary, located beneath an abandoned house in Cheydinhal. To enter the sanctuary, pick the basement lock and respond to the question "What is the color of night?" with "Sanguine, my brother"

5. Meet the Brotherhood and Begin Your Questline
   Inside the sanctuary, you will meet the branch leader Ocheeva and other members. You will receive your first contracts and a suit of shrouded armor, marking the start of the Dark Brotherhood questline

This process emphasizes stealth and careful selection of your first victim to avoid complications. After joining, you gain access to unique assassination missions and rewards exclusive to the Dark Brotherhood faction.

Summary

• Murder an innocent NPC (not essential or hostile).
• Sleep in a safe place to trigger Lucien's visit.
• Kill Rufio at the Inn of Ill Omen as a test.
• Sleep again and gain official membership.
• Enter the sanctuary in Cheydinhal by answering the secret phrase.
• Meet Ocheeva and start the Dark Brotherhood quests