If you descend or if you descend a staircase, you move downwards from a higher to a lower level.

If a large group of people arrive to see you, especially if their visit is unexpected or causes you a lot of work, you can say that they have descended on you.

When you want to emphasize that the situation that someone is entering is very bad, you can say that they are descending into that situation.

If you say that someone descends to behaviour which you consider unacceptable, you are expressing your disapproval of the fact that they do it.