Why does Riku fight Sora in The Land of Dragons?

[DarkGrey Heroine]

the thing that needs needs be answered is why riku's keyblade has that eye on it that xanort and venitas's keyblades have. could riku be a dark vessal

To clear things up, here are all the keyblades with these so-called Eyes of Darkness:

There are also some Keyblades from KH Chi with the Eye, which is interesting enough, but since I'm totally new to KH Chi, I won't talk about it that much ^^;
(and there's also Terra's Darkgnaw but I honestly don't see the exact same Eye in this keyblade's design, so I didn't mention it...)

The Eye of Darkness appears on Keyblades related to the Darkness in one way or another.
Of course, that means the one wielding the Keyblade has a certain affiliation to the Darkness as well.
Now, about the Eye's origin... Remember our One-Winged Angel, Sephiroth? From Final Fantasy? The Eye of Darkness is inspired by his eyes:

The Kingdom Hearts series connects with the Final Fantasy series several times and FF symbols and characters appear in KH... I think the Eye of Darkness is a cool thing and it's so well-chosen to be inspired by Sephiroth's eyes, because if Sephiroth doesn't represent the Evil or the Darkness then I don't know what would jeez xD

I'd like to explain the "if Riku is a dark vessel" thing too, but I don't want to spread spoilers ; ^ ;

 

[Riku-KH]

That's what I've always thought too. Kh chi confused me though. May be because I don't know anything about it! Were all the Foretellers dark or something? That does seem pretty coincidental that with Sephiroth's eye looking like eyes on keyblades. But would they make kh be influenced by ff that much? That doesn't sound right... I also think it's so strange that Sora optains end of pain in DDD like Riku does. Did he get that close to the darkness in the dream world? It seems that way. Everthing for Sora was different in that game.

 

[DarkGrey Heroine]

That's what I've always thought too. Kh chi confused me though. May be because I don't know anything about it! Were all the Foretellers dark or something? That does seem pretty coincidental that with Sephiroth's eye looking like eyes on keyblades. But would they make kh be influenced by ff that much? That doesn't sound right... I also think it's so strange that Sora optains end of pain in DDD like Riku does. Did he get that close to the darkness in the dream world? It seems that way. Everthing for Sora was different in that game.

Well, Kingdom Hearts mixes the worlds of Disney and Final Fantasy together and symbols of both appear more or less in the series. It's no surprise to see FF references here and there. The influence of FF is pretty big, think about the omnipresent Moogles, the Radiant Garden world, Keyblades like Fenrir and Metal Chocobo, some of the Magic spells and Attack commands... Here's an article covering some other similarities, though not all of them: Kingdom Hearts - The Final Fantasy Wiki has more Final Fantasy information than Cid could research
Nomura developed rather complex series, right? Everything has a meaning...

DDD sure was a different game, in which our very Hero of Light gave in to the darkness. This means Sora was indeed close to the darkness, thus End of Pain has the Eye of Darkness on it.