The first line in your description is what shows up next to the thumbnail in search results. So, I try to give an extra reason to be interested in my video if the thumbnail and title don't do it. For me, that line is always "I take requests! Please watch until the end and leave your idea in the comments." If a horror version of something related to what they watched or searched for isn't quite enough to interest them, maybe the idea that I take requests will make them think that there are more where this one came from and they also can have a say in what other ones I do. And then maybe they are interested in just what it is I do.
As far as the rest of it, I usually try to fit as many keywords in as I can in a reasonably-structured couple of sentences. The last paragraph is also the same on all my videos and it describes in general what the theme of my channel is. These settings are set as my upload defaults, so they are already there when I start a new upload.