There's a prophecy from who knows when that's brought up by Urianger in 1.0 that talks about their being more then 6 Umbral Calamities. Most people since the 6th Umbral Calamity discounted it since there'd been six Calamties, one for every element which was nice and neat. Obviously, that's not what happened. The Calamites just keep coming. It looks like the knowledge of there being future Calamities goes back a long ways.
Part of the reason the Onishishu are even there to begin with is because there has always been people who try to stop Calamities from happening or mitigate the worst of the effects. The Onishishu are offshoots of what looks like to be good contenders of the 4th Calamity's Warrior of Lights, or at the very least, people who probably had the Echo. Their way of paying homage to past and future WoLs is to give them a good place to train that didn't come out of some aweful circumstances. Like, most of the "getting stronger" the WoL has done has been though stopping bad event from happening. This is pretty much the opposite and is one of the few glorified practice halls that are supposed to be practice halls.
It's mentioned that fighters who aren't found worthy by Hiruko, the Lvl 30 boss are killed by it. The fact that you, the WoL are alive means you passed the test. So have all the leaders of the Confederacy. Presumably it's there to make sure all the fighters that don't agree with the morals of the Onishishu don't ever leave the tower.

Originally Posted by
Kyusei
Yes, like Rasho and the pirate captains before him, you bested Hiruko. He looked into your soul and found you worthy. Were you found wanting, well...I am sure you can guess the fate of those who do not pass muster.