I just wanted to point out that they talk about how Zed is to become Nocturne. And if this is true, is it possible that he becomes Nocturne once he dies, he does crall into a dark portal, maybe succumbing to the Darkness as a last desperate atempt to survive. I think Riot planted this as a seed for us to find, but will never add anything on to confirm it. Just a fun easteregg not part of the lore really. 
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ModWulf Helhammer1 votesThis has already been discussed.
Arakadia1 votesWell I havent seen it.
Ebonmaw Dragon1 votesDarkness and Shadows are different things, a shadow is generated when the light is blocked by an object... darkness in other hand means that there is no light at all... a shadow cant exist in darkness.
And Nocturne is a living nightmare with darkness powers... not a shadow. Zed has no relation to nightmares at all.
Punkkee1 votesso if im reading between the lines zed isnt suppose to be able to use his shadow when nocturnes ulting :O zed hax report lel gg op
DarkFyreZ1 votesXD Punkee has a point. But I agree with the creater of this post... I think its just a fun easter egg.
Arakadia1 votesYou want to get nit picky and scientific? Well, A: we're playing a game where little girls burn people and ancient emperors come back to life to summon soldiers made of gravel. And B: Its all about perspetive. What we call "pitch black" still has light just a very small amount. Thus shadows can still could exist in "pitch black". Another creature may be able to see these faint shadows because they have more sensitive eyes that can absorb light. Darkness is also described as the partial or total absence of light. And doesn't a shadow create darkness. Thus the opening of the box, Zed's usage of the shadow arts, and, as a said in the theory above, Zed using a dark ritual to save himself from death may have been the theoretical shadow that creates the theoretical darkness known as Nocturne. Technically the only reason darkness exists is, as you said before, a shadow is when light is blocked by an object, thus creating darkness. And we just call Nocturne a nightmare because thats a term for a monster. In history people have discribed animals and people as "nightmarish" but does that mean their creatures of myth? Your right though! Zed has NOOOO relation to nightmares at all. He is completely related to shadows, darkness, corruption, and both Nocturne and Zed have blades connected to their arms. If you dont like these kinds of responses, try not to be smart-alike about how someone else is wrong.