Aside from legendary creatures with obvious origins (due to being in a plane-themed set) this thread is created for legendary creatures without very obvious origins.

If anyone has trouble understanding where a legendary creature came from, or has an answer to another user's inquiry, feel free to post that information on this thread.

I personally wonder where Riku of Two Reflections originated. My first guess is Kamigawa, but it's a long shot.

Shandalar actually.
We got that bit of info well after his introduction though.

Lemme run it down for you, though it might take a bit.

edit: http://magic.wizards.com/en/story/planes/shandalar

I think the clarification came from one of the Duels of The Planeswalkers games, but I can't say for certainty at a cursory glance.

On a somewhat related note, it's awesome that the flavour text on Command Beacon quotes Riku in that EDH meta kind of way. There must be a cooI story behind that beacon and whoever set it up. I want all the Shandalar...

Getting the two halves of Riku on separate cards with Partner would also be really cool if they ever find a place for that.