Wtf is this?

This is one of the Costa Farms euphorbias that I bought this past summer. It’s been inside for about a month and a half if that makes any difference. As I was looking at my plants yesterday I noticed this discoloration. Is my plant dying? Is it infected with something? I don’t see anything on the plant or the soil that looks like bugs. It looks to me like it’s rotting. I haven’t watered since bringing them on as we had a huge rain storm right before that and I figured that soaking would last them for a while. What do I need to do?