Looks grim, probably rot!
Soil looks very rich, too. Remove from pot & check roots & neckline. Others will probably jump, but looks like cut-thru with sterile knife. Cut slices, working towards tip to see if if any of it "clean", i.e. healthy, to dry & save as a cutting. I'd be cutting potting mix with @ least 50% coarse grit, Turface, NAPA/AutoZone, vermiculite, something...to reduce organics & increase drainage.