What Is The Callus With Black Spot On It On The Heel?

hi, i am about to be 23 and i have had a callus on the outer right sideheelof my left foot since i can remember. lately ithas been hurting a lot more and i have a black spot on it. when i remove the dead skin it becomes tender and hurts more. also the spot opens and i see its a whole with a thick black liqquid. i have decided to not remove the skin. what is this and what can i do about it. i am 5 feet 2 inches and weigh 140 pounds