....I love how just a few weeks ago everybody was shooting down my Surtur theory completely, and now you guys are jumping all over it.....bandwagoners.
That being said, there is another slight possibility to consider. It's a very obscure character, from back in my day, but the description seems to fit perfectly: predates the Universe, too powerful for even Thor and Odin to take down, and his main power involves darkness.
The Dweller-in-Darkness first appeared in Thor comics; and later became a recurring enemy of Dr. Strange. Which would also lend credence to the Dr. Strange casting rumor.