I recently repotted this old cactus, a gift for my birthday, and it is very unhappy and getting very brown from the bottom. First images are from a week ago, last ones show that it is getting more brown since then. Have not watered it in 2 weeks, watered it very little before then. It is in a sunroom with lots of direct light. Still has new growth on the tops. It was happy before the repot, but now in decline. Any advice? Thank you!!!
Unfortunately at least two appear to be rotting as can be seen from the discoloration moving up from the soil line. If it is rotting there is very little that can be done other than cutting off the healthy part, letting it callous, then trying to root it as a cutting. The diseased portions should be properly disposed of (trash or burn it, don't put it in your compost) to avoid spreading the disease. The likely cause is a minor injury during the transplant allowing the disease organism into the cactus. Here is a link with the steps for dealing with a rotting cactus: https://www.hunker.com/12603876/how-to-save-a-rotted-cactus
It is not a cactus but an Euphorbia (ingens?). Not sure what is going on there, the brown areas on the lower parts looks like lignification (which is normal), but the rapid spread is worrying. Or would be worrying if it had been a cactus, I don't know that much about Euphorbia. Yellowing skin can indeed indicate something deadly, but can also be caused by malnutrition. The plant is definitely not well, and the fact that the changes appeared shortly after a repotting makes me suspect that it can be something really bad, much like what Will B wrote. This species can grow to a considerable size, so perhaps you could sacrifice one stem, cut it up and see what is going on on the inside?
