Okay, they look just like birds. A flock of them will generate electricity, passing it through themselves. I don't know how or why. They've been known to cause lightning strikes. They are probably the reason for the myth of "Thunderbirds" (not the Gerry Anderson show though).
Like the Slender Man, they have proxies. They are called Nests. And there is a very good reason for why they are called Nests. Take a guess.
That's right, they are nests for the Convocation. The Convocation nests within a person's body and they become a Nest. I don't know what sort of special powers this conveys (does it convey any powers besides creepiness? no idea), but they are dangerous. They can cut open their skin to let out birds of the Convocation and the birds? Will mess your shit up. I have seen a flock of birds carry away a full grown man and drop him from a great height so that he ended up just a splatter on the pavement.
Supposedly, they come from or travel through a place called the "Bleak Shore." I have no idea if this is true or bullshit someone made up.