Hello Tedgar420, to me it looks like it sat on a wet media for some time while weather may have turned cold and plant was dormant. Was this grown outdoors or indoors? There is also some natural corking occurring as the cacti ages. The first and second photo shows also it may have some rotting at root level already, so maybe base below soil level is rotting. I would pull that out and inspect that part, if it is truly rotted out, do some surgery, remove the rotted part and set aside to callus and dry. Apply some light cinnamon on the cut area for protection from fungal issues.If surgery is done, it takes a long while for the plant to recover and do some new roots, it just depends on your ambient growing conditions there, so protect it from getting wet while in recovery.
I would suggest you add more pumice to that media and make sure surface is much grittier so the plant is not sitting in any soil that will hold moisture. If your weather permits it outdoors, it is best to bring it out, allow some part sun to full sun. If you intend to grow it indoors, have to position it by your sunniest window, ideally south facing.
Double check if container has drain holes, and wear gloves as you handle this plant.
