The idea behind it is that the Great Old Ones are immortal and simply biding their time until they are released and/or escape imprisonment, and when they do, they will destroy all of existence, including death itself.
For a watered down and very nearly plagiarized account, just look at the old gods and void lords in wow.
Cheerful lack of self-preservation
"That is not dead which can eternal lie"
The order of words are, in multiple ways, backwards. So let's reverse it:
1) "that is not dead" = it ("that") is alive
2) "can eternal lie" = can sleep ("lie") forever ("eternal")
3) Even the order of the sentence itself can be reversed...
Therefore:
"That is not dead which can eternal lie" = If it can sleep forever then it is still alive.
Would you all stop lying? What is this a Mafia discussion?
Next mafia should be Cthulhu themed, imo.
https://www.youtube.com/watch?v=cUEUF6Qh8ys
Cheerful lack of self-preservation
Nothing beats https://www.youtube.com/watch?v=23cjXModWpA