The Orokin feared nothing more than death, ugliness, and the end of their rule. The situation of overpopulation and ever-exceeding consumption of resources was reason enough for the Orokin to create the Sentients, terraforming machines that were highly adaptable and regenerative. Not to kill the Tenno outright, but to send the Sentients instead to Tau in their stead. However, the Sentients grew sentient. Poetic, isn't it? They knew the Orokin would plunder Tau and drain it of all its resources, so they returned from the Void to wage war against their masters. But the Sentients were also angry since they discovered that exposure to the Void rendered them incapable of reproducing; a weakness that would leave them barren and childless. This is when the Old War began. And with the Old War came the Warframes, the surrogate bodies you control in the game to wield electricity or rain fire and brimstone down on your foes. But the Tenno wielded the one element the Sentients couldn't adapt to, the Void. And so, the Tenno fought back against the Sentients and brought an end to the Old War.