The EXPCalibur is a mighty two-handed sword.
It can be found in the bottom right corner of the slums, on a rock in the water. The sword does massive damage, but unfortunately can only take 7 hits before it breaks.
When you have located the sword you must hold down Square (PS4) / X (Xbox One) for a couple of minutes to pick it up. Removing the sword will start a little fire on the rock. Wait until the fire has burned out to pick up the blueprint for the EXPCalibur.