Zed will only use nightfall in these following condition...
-You QSS Zed ulti, he use nightfall after you QSS soo it makes up for no ult -If you are a Tank he will use nightfall to oneshot you -If you zhonya Zed ulti, he will use nightfall after the 2.5sec to redo his combo That item is like having 2 ult on a champion...