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
https://imgur.com/a/Z0rpudr
You will need to go to the second floor of The End of Time and talk to this guy:
https://imgur.com/a/8EtpYKp
You will enter the portal and run to the end and talk to all 3 of these guys:
https://imgur.com/a/Umc9mnZ
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