The quest is also bugged atm and yes A LOT of mobs will spawn. Not really sure how to by-pass this. I personally die and they all disappear. OBVIOUSLY this wont work for HC.
You will first need to complete the first Era and get to The End of Time
You will need to go to the second floor of The End of Time and talk to this guy:
You will enter the portal and run to the end and talk to all 3 of these guys:
When you leave you will be given the choice of one of the masteries your class has. NOTE: this can not be changed later.
You can take passives from the bottom half of the other talent trees though