The maelstrom would be a fine example sir. Other notable examples are noted in any book where the author needs to explain why the will be daemons in the next few chapters, "the fabric between our universe and the warp must be thing here, I can feel things scratching, dark things, trying to get through".
Thats not a quote from any particular publication, but is so close to so many actual quotes, take my word for it.
"The difference between gods and daemons largely depends upon where one is standing at the time." - Aurelian Lorgar