Those spots between ribs look very water-logged: yellowish, watery (almost translucent) color. But photos are sometimes misleading. Can you somehow touch it - maybe use a pencil and see if it 'gives' easily? If so, it could be rotting. In that case you would need to perform surgery - cut off all of rotting stem/trunk and try to reroot healthy reminder. If it is rotting and left as is, it will eventually die. So attempting to save it is better than not , even if success is not guaranteed.

Your mix looks like it has peat in it - does it? - and not enough perlite or pumice. If using either of those, you need at least half of the mix (could be even more) to be of inorganic substrate, and less soil.

As for growing 'babies' - it may or may not. Most important would be to save it first...:)

General info for all succulents: fast draining potting mix; container with drainage holes; watering when needed; plenty of light; good air circulation.

I forgot to mention about touching it. I do brave the pricklers and regularly touch the skin, always firm. I know it looks like it’s rotting and that’s what I thought at first, but I honestly think it’s severely dehydrated. It’s looks slushy, but it’s really wrinkled.

Soil is a cactus mix that is heavily amended with perlite.

Here is photo of mix that Crenda uses (and many others)L

I too am concerned about the discoloration working it’s way between the ridges. I’ve been trying to keep an eye on it for the last three months to see if it’s getting worse and I’m just not sure if it’s changed at all. It’s significantly higher on one side, than the other. It’s so high, I’m not sure if there’s enough left to save. Maybe two and a half inches at the top don’t have it.

Its always looked bad. But I’d go years without seeing it, so I I’m not sure how bad or any details.

I really do want to chop if off in an attempt to have it grow back healthier and stronger, with a better shape. What will happen to the bottom half if I cut the top off? What advice do you have for successfully saving the top?

Side question: I have these 3 babies. They’re too tall and thin to be barrel, right? Anybody know what they are?

Your Barrel cactus I would cut right above the parts that look rotted. Use a clean sharp or serrated knife. When you cut the cactus check the flesh on the cutting. If there is no brown or rotten parts you don't need to cut anymore. If there is even just a little brown sdot keep cutting till the flesh is clean. The cactus needs to form a callus or scab which can take 2-4 weeks. During this time keep in an area that has filtered bright light, warm, and dry. No water till roots form. If you live in a humid area you may use a small dusting of cinnamon on the cut. This acts as a natural fungicide;).

Once a hard callus has formed repot into a mix of 60% rinsed and strained perlite or pumice and 40% cactus mix. Don't go checking for roots once you've potted your cutting up. This rooting process can take a month+. The constant disturbing will slow this process down and you risk ripping out newly grown roots. Just keep the cutting warm, dry, and in bright light. You will know when roots have grown, your cactus will plump up and start looking good again. When roots are present you can begin to water and acclimate to more light.

The bottom part of the cut cactus you can treat like a regular cactus. As long as there is no rot present. When watering always make sure the mix is thoroughly dry. You can do this by taking a wooden skewer and dipping it all the way down the container. If that skewer comes up damp anywhere then wait to water.

Goodluck;).

Whole idea of cutting top off is because it looks like it is rotting. If that is the case, there is no bottom part to be saved...

If bottom was healthy enough and was cut, it will probably start growing one or few 'arms'.

I know, I want to save it too. I was just thinking that if the top doesn’t make it, maybe the bottom will sprout a baby, that I could try to root as a consolation.