Well, it usually takes a LOT of luck. You can only find the World Eater Blade on Isla Tormenta or Raven's Cove. Legendary weapons are extremely rare, you need to get really lucky before you even get a famed.
It took me a few months at Foulberto Smasho to even get the Silver Freeze. With luck and lots of praying, you can get this weapon. When you find the World Eater Blade, you will be so glad you did. It has a freeze sweep and an attack of one-hundred-one.