this should be in the main ladder slasher forum but oh well. clan discussion topics go here.
But basicaly as you hit level 71 you will gain the ability to do the Master Quest and if passed you will unlock the new secret character. When you pass the MQ with the new unlocked character then another one will unlock. when you have unlock and completed the master quest with all of the unlocked characters then you will reset the whole season and everyone will go back to being level 1 and having to start from square 1.
Thats basicaly it. Not gonna say WHAT the master quest is and what you do in it as that is something for you to find out yourself.