Maybe you understand reliance differently, but for me if you use something for your advantage, you rely on it to give you advantage. If I use a screwdriver to drive screws, I rely on it to drive screws (whatever it means). If I use a crowbar, I rely on it to do the job. Without that tool I'd either be unable to do the job, or have to find an alternative (my hands for example).
So far no one had found an alternative to Time
, or at least no one still alive, muahahahhaha 
As for eternity, being eternal requires Time. Without Time nothing can be eternal. It's similar to saying that without distance nothing can have an infinite length.
Beings that exist outside of Time are not really eternal, it's a state that has no word to describe it in human languages. The main difference is that eternal things still have past (infinite), present and future (infinite). For beings outside of Time the words past, present and future have as much meaning as words Gandalf, Sauron and Gimli. It is something that from their point of view might exist, but in another world, or might not exist at all. It doesn't affect them at all though. If they have a way to influence other worlds, they don't give a damn if it's in the past or future from our point of view.
As for darkness, in our world it might be eternal because outside of time-space continuum there is no darkness. If the universe ever comes to an end, Darkness will also end.
Though, in a more metaphorical/metaphysical meaning you might extend the meaning of Darkness. For example calling things that are 'unknown' dark, or calling the act of stealing and draining powers to be of the Dark ways. As long as these acts are timeless acts, which is a very complex concept from Earthlings logic point of view, they might exist outside of time-space continuum. It's similar to the concept of God creating angels and other daemons 'before' he created the universe. There is no time relation between these events, but there is a cause and effect relation.
