Zenitsu mission requirements

The Zenitsu mission is the quest line used to obtain the Thunder Sword. A current player walkthrough reports that you need B Rank or higher and $1,000,000 before the Sleeping Slayer NPC will let you progress.

Because Legacy Piece changes quickly, read the NPC dialogue in your server before beginning. The live quest tracker is the final authority if a later patch changes a number.

Step-by-step route

  1. Travel to Slayer Mansion and speak to the Sleeping Slayer NPC.
  2. Defeat 75 Demon Slayers.
  3. Complete the breathing practice by blowing the small gourd 10 times.
  4. Blow the medium gourd 25 times.
  5. Blow the large gourd 50 times.
  6. Return to the Sleeping Slayer NPC to claim the Thunder Sword.

How to avoid wasting time

  • Reach B Rank and save the full purchase amount before travelling to the mansion.
  • Keep the quest pinned so every Demon Slayer defeat counts.
  • Finish each gourd stage in order; do not assume progress on a larger gourd replaces an earlier task.
  • Rejoin an updated server if the NPC or quest counter does not appear.

The steps above come from a current gameplay walkthrough, not an official static quest page. Verify them against the in-game quest text before spending money.

Source: Zenitsu obtainment walkthrough.