you will come across star stones in main quests, and so by lvl 21, the hall of heroes should already be opened. Zixzax will always stand by the portal to be opened. if you have no portal to open, he will stand besides the time weaver.
and of course if you didn't open any portal before the closing the rift quest is activated, you cannot open any until the bloodstone is destroyed.
as for the bloodstone
send one of the main characters to destroy it, and then use a bloodstone to cure the rot
then, you can open all portals and proceed to end game.