The Stalker randomly shows up and tries to kill you.
There are three ways to tell if all your Dens have a Master. Firstly, check to see how many of your Assassins are higher than level 10. While all Assassins will still gain EXP once they reach 10, only those in charge of a Den can achieve 11 or higher. so if you've seven Assassins higher than level 10, all Den's have a Master. If only some are level 11 or higher, then you still need to assign Assassins.
Another way is to look at your map. Any Den with a Master will have a slightly different icon that just adds a few lines around the corners to give it a box shape.
The final way is to check DNA and see how many Part 1 missions you've done.
Last edited by SaberBlade; 12-01-2011 at 02:46 PM.