If the Elder One has been influencing things since before the fall of Arlathan, than we really are in some deep ****.
Let's look at the things he would have had at least an indirect hand in:
1. Humans learn blood magic
2. Destroy the elves
3. Enter the Golden City
4. Blight (Destroy the world)
5. Ferelden Civil War
6. Anders/Justice blows up the Chantry
7. Meredith goes Ape ****
* this is just the few I could think of*
If we go off the theory that the EO can 'whisper' into peoples dreams and amplify and suggest things (perhaps even "speak" with mages since they have a greater connection to the fade) then the EO could subtly influence things. He doesn't directly cause things but instead slowly moves pieces to fit what he wants to happen.
If this is true than we would be dealing with a being who has been plotting for ages upon ages. That kind of patience and cunning is crazy scary. He has been watching the world, learning our weaknesses and how to manipulate us for a long time. Meaning from the very start we are up **** creek without a paddle, so yeah crazy scary.