To get the Dark Dagger in Blox Fruits, go to the Third Sea , summon and defeat rip_Indra True Form at Castle on the Sea, and hope for the drop. The Dark Dagger has a 5% drop chance from that boss.

Steps

  1. Reach the Third Sea.
  1. Get a God’s Chalice. You can obtain it from chests, Elite Pirates, or by praying at the Gravestone.
  1. Collect the three Legendary Aura Colors : Winter Sky , Pure Red , and Snow White.
  1. Go to Castle on the Sea and activate the three aura buttons.
  1. Place the God’s Chalice on the pedestal to summon rip_Indra True Form.
  1. Defeat him and roll for the Dark Dagger drop.

Helpful note

Buying 2x Boss Drops can improve your odds over repeated runs, since the sword is still a rare drop.

Quick tip

If you’re farming it with friends, it helps to have the aura colors split across players so you can summon rip_Indra faster.

TL;DR: Unlock Third Sea, summon rip_Indra at Castle on the Sea with God’s Chalice plus the three Legendary Aura Colors, then defeat him for a 5% chance at Dark Dagger.