Hmm.... well, one should certainly not tell the players... however, there should be a reason, a conjunction of the stars and prophesies, as to why not just individual souls, but a whole gang of heroes, gets reincarnated as a new band of heroes... do they know what the previous heroes of old did? Maybe the evil they fought has returned.
Maybe however, the tales of old have been altered throughout the tellings, to the point that many references in the tales are not literally true, but merely mythological pointers. Maybe the story got twisted so bad, the 'heroes' were not really heroes at all... they were the evil sorcerous kings of old. Now their souls are back. For redemption... But... some other agencies have also read the prophecies. And misunderstood. They think the PC's are indeed the ancient BBEG's, but that they want to retake their lost thrones. There are 'evil' agencies that offer their aid (or help in the shadows) and at the same time, there are 'good' agencies actively trying to hinder or even kill the PC's...
It will take some time before the PC's understand what is going on, and even more time and effort to convince the good guys they are good guys too, while the evil guys will turn on them with a vengeance and will keep trying to turn them 'to the dark side'...
Just an idea...
