I can't really say what happened to the Seven Seraphs, but Osiris is no longer a 'faction' like the War Cult, New Monarchy, and Dead Orbit. Osiris is apparently an ancient Warlock that is both wise and powerful. He disappeared from the Orders, and from known space. The best the Tower could find was recordings and mere traces of where he's been. There's an as of yet unannounced event called the "Trials of Osiris' for which you can only access with a tournament ticket or an Osiris coin (no one knows how to get those). It's a crucible event that only takes place on the Burning Shrine on Mercury, and if you lose three times, you need to purchase another coin in order to get back in. you can get some legendary gear apparently, but like I said, no one's played on it.