ok so this is basically what happened during your absence.
the 3 knights we see in kh2's ending are all connected to sora riku and kairi, their names are:
Terra(the tallest soldier): means earth
Ven(the younger soldier): means air or wind
Aqua(the female soldier): means water
you can see all of their faces on youtube, here is a link
YouTube - Kingdom Hearts 2: Final Mix+ Secret Ending [AUDIO AND SUBS]
so the old guy you see is the U.E.M, unidentified elder mage or wutever you wana call him, and he splits into 2, the D.S, the dark soldier, there have been many theories on who these new guys are, one has been that the uem is xehenort or a decendant to xehenort and he had researched darkness and htings and figured out how to split into a darkform while still keeping his original form. people also think that he is a nobody or sumthing.
i personally think that when the 3 knights defeated him, he split into 2 again and gave xehenort his powers but those powers lay dormant.
also, if you look closely, you will see that the UEM is the only keyblade weilder who has a keychain on hte end of the keyblade. there are also many theories for this to like hte 3 knights have a noob or pro version of hte keyblade and hte uem is advanced or noob or sumthing.
also, there is much speculation on wether teh final mix+ will be released in N.A or not. and hte vid you are seeing, is the ssecret ending vid to the kh2fm+
if the game is released here, a disc version of CoM will be realsed with the fm+ being called re:CoM
also in the vid, you will see mickey appearing, we think that mickey alraedy knew about the threat long ago and didnt tell sora and hte others for some strange reason.
oh and by the way, the secret ending took place in the past.
the 3 knights are also connected to sora riku and kairi, if i havent mentioned that before,
sora=wind-ven=wind
riku=land-terra=earth
kairi=water-aqua=water
also, there also has been speculation of why sora was given the keyblade when terra, in a special boss fight, said that he didnt choose sora.
sora is hte leader in their group and terra is in hte other
we think that riku was supposed to get the keyblade but it was given to sora instead because riku accepted the darkness.
well, i think thats about it... if i missed something, im sure someone will add it later! i hope this helped