According to the search results, Greybeard is a level 750 raid boss in the game Blox Fruits, located at Marine Fortress Island in the First Sea. He spawns every 6 hours after being defeated and players have 15 minutes to defeat him before he despawns. Greybeard is a pretty strong boss in Blox Fruits, but he drops not so many rewards. All you get is leveling up and the opportunity to upgrade your Bisento to Bisento V2. To do this, you only need to have a weapon in your inventory and deal at least 10% of the total damage to the boss. Therefore, Greybeard does not drop any specific item or reward other than the opportunity to upgrade your Bisento.