To get the Soul Guitar in Blox Fruits, you need to start by reaching level 2,300. This is necessary to begin the Soul Guitar puzzle. Next, wait for a full moon in the third sea; you will see a message on your screen indicating when it’s a full moon.
Once it’s a full moon, head to the graveyard side of the Haunted Castle. Look for a gravestone with a white circle around it and click on it. After this, gather six living zombies on the right side of the Haunted Castle and kill them all at the same time. Using a fruit like Buddha or Light Fruit will make this task easier.
Return to the graveyard and count the graves on each side. Click on the side with more graves. If both sides have an equal number of graves, do not click anything. Next, head to the other side of the castle and take a screenshot of the trophy patterns. Copy these patterns to complete this step.
Now, you need to gather some materials. Collect 500 bones by killing zombies in the same room where you completed the previous puzzles. To gather 250 ectoplasm, use the Buddha fruit and unawaken it at the cafe to avoid obstacles. Lastly, get a Dark Fragment by killing Dark Bid, which you can summon using a Fist of Darkness.
Once you have all the materials, return to the NPC and craft the Soul Guitar. Enjoy your new weapon in Blox Fruits!