The Ebon Eyes spell simply makes the beneficiary see shadowy illumination as if it were brightly lit. This even works with spells that cause low-light conditions, such as Darkness. This does not stack with nor otherwise extend/double normal low-light vision ranges.
This is different from the "Low-Light Vision" spell (and natural low-light vision itself) because that type of vision does not upgrade any existing lighting -- it merely increases the distance to see well. So someone with low-light vision in an area of shadowy illumination does not see it as brightly lit; they see it as shadowy. It is only when a bright light source (such as a torch) is introduced that low-light vision kicks in, increasing the distance they can see well. However, with Ebon Eyes, a person in an area of shadowy illumination sees as if it were daylight, which removes the miss chance that low-light conditions normally cause in combat. Since sneak attacks are normally not allowed if there is a miss chance, this opens up the "thug in a dark alley" scenario for rogues.
This has no effect on "total darkness" lighting conditions. It offers no counter to non-darkness problems. If you are blind, Ebon Eyes won't help. If you are in fog, Ebon Eyes won't help.
Technically, this creates an interesting vision effect when gradated lighting applies. For example, a normal human carrying a torch through a dungeon would see bright & clear for about 20 feet, then shadowy illumination for another 20 feet beyond that, and finally pure darkness after that. With Ebon Eyes, that same person would see bright and clear for about 40 feet, then pure darkness. There would be no in-between shadowy illumination.
