What does "a descent of night" man here? Does it mean the dominance of night?
"It is the very event of observing, a descent of night, an invasion of shadow." mean?
In English we associate day with rising and night with falling, perhaps because that's what the sun does during those times. So if you want to say "It is night time" but you want to be a little more poetic, you could say "Night has fallen." Descent can take the place of fall in this context, so your sentence means, literally, "It is the very event of observing the transition from day to night."
We also sometimes use night figuratively to mean evil or death. I don't know the context of your sentence, but the author probably refers to either the transition from daytime to nighttime, or the transition in a person or culture from good to evil or alive to dead.