Re: Why dkes Riku's keyblade have an eye?
Eye of Darkness = some kind of connection to darkness, it appears on the wielder's Keyblade if he has or just made some connection to the darkness. I wouldn't go as far as making is a symbol of some kind of connection to Xehanort. The golden eyes are a proof of that sort of link already.
End of Pain has the Eye too, and it's rather the representation of Sora realizing the darkness and hurt within himself that he must solve. You obtain it after all the DDD plot, so it's after the connection between him and the darkness has been made.
I guess the Void Gear has two Eyes for the sake of a cool design, also because Vanitas is pure darkness and nothing else.
Sora only went through that pain because Xehanort forced him to. He nearly became a Xehanort himself. KH3D's entire plot is about how Xehanort has been controlling their fates. It can definitely represent the Darkness (and that was likely the intended meaning when this series started) but there is no way to not connect this to Xehanort anymore when every instance of this eye's appearance coincide with something Xehanort has done. We never see this eye motif appear under any other circumstance except when Xehanort has manipulated things for his own plans.
SoD - His spear is pretty much Soul Eater.
Riku - In Re:coded it is discovered SoD met with Riku as Destiny Islands was being destroyed by the Darkness. Whatever that meeting was about, the next time we see Riku in the first Kingdom Hearts after the Rising Falls scene, he has this weapon. It is never stated how Riku learned how to use it and we always assumed it was Maleficent who gave it to him, but considering what we know about SoD's machinations, it seems likely that Riku obtained it due to the fact he met Xehanort.
Later, Soul Eater becomes the base for Way to Dawn and Riku can summon them interchangeably. Note that Riku gains his new Keyblade while still under the guise of Ansem: SoD.
End of Pain is obtained after completing all Special Portals, but due to the name it seems to be a representation of what Sora and Riku had to face in KH3D and all of that was controlled by Xehanort.
Terra - Darkgnaw and Chaos Ripper are Keylades we obtain as Terra. Chaos Ripper, being the only story Keyblade, is obtained after Xehanort sets Terra up and kills Eraqus. It is a representation of Xehanort's manipulation of Terra, of Terra's Darkness that was only forced out of him because Xehanort made it so, and it can even represent the fate that befalls him at the end of BBS.
Vanitas - We know that Xehanort created Vanitas from Ven's Datkness and Ven had been a planned vessel before he proved himself to be too frightened by the Darkness inside of him. Vanitas then becomes Xehanort's vassal who does his dirty work and also serves as a means to the end in order to create the X-Blade. Due to the fact that we now know the gold eyes do not represent Darkness but rather that person has a piece of Xehanort's heart within them, we can pretty much assume Vanitas probably did too. His brief appearance in KH3D has led many to this theory.
Sora - End of Pain, although not being a story-based Keyblade, still has its own meanings. Sora was led and trapped within his own Darkness by Xehanort. This Darkness was caused by the pain of those Sora is connected to and the fact that Sora attempted to take on that pain at the wrong time because Xehanort forced him into a dream within a dream. That Darkness made Sora a prime vessel for Xehanort, and if Lea had not arrived when he did, Sora certainly would have been.
Now, if the eye really represented Darkness, I believe we would have seen more instances of it being used that were unrelated to Xehanort but we never have. Xehanort has been our main villain, yes, but Maleficent and other villains who utilize the Darkness have been in the series to give opportunities for it to appear and yet the eye motif has been predominantly connected to Xehanort and his connections to the story.
In the past this might have been true because of the fact that the Foretellers have them, but that is more like an origin point for the eye and not its modern use and meaning. Xehanort's Keyblade, being the oldest, comes from this time so there is clearly something going on with that and we all recognize that.
But with every moment shown within the series where the eye has been present, it has always come back to Xehanort directly and indirectly.