Unhelmed and Unyielding

At the beginning of your upkeep, put an amass counter on Unhelmed and Unyielding. Then, for each amass counter on Unhelmed and Unyielding, exile the top cards of your library. You may cast cards exiled this way this turn.
At the beginning of your end step, you lose the game unless you have cast each nonland card exiled with Unhelmed and Unyielding.
"With no one to lead them, any death is defeat."