This is fungal. think your plant is done for. This seems to from the inside out

You may have very water retaining soil. I would take plants out ASAP. You can try cutting the top off, well above the black part. See if the flesh is clean. If you see any black/dark (spots, line or anything), it should be cut further. Still, no guarantees. Sooner - better.

And definitely pot up other succulents into new mix. Should be well draining, and pot should have drainage holes.