How was the Darkness imprisoned?

YE know that it is right that Mani be asked: From which of the Elements was the Grave built for the Darkness? But if it spontaneously turned and imprisoned itself, know that, because it cannot mix or mingle with itself anything else --  [P. 91, l.20.] for there is nothing -- and because, moreover, it cannot change itself -- for it is an (eternal) Entity which exists as it existed before, and does not come to change -- it cannot become opposed to itself. But if he built (the Grave) from the Element of the Good (Being), how did he make it from these Souls in whom he takes delight to-day? But if there is essentially belonging to his nature something which is harder than these Souls, then why did the Darkness not build from that hard and deaf (i.e., inexorable) and victorious element a wall for the outer Domain in order to keep his possessions within? And thus [P. 92.] he would have been spared all these evils. But, perhaps, this wisdom had not come near him at that time, but in the end (?) of his years it happened that he was harassed and learned,, practical (?) workmanship and stone-cutting, and architecture. . . . And if these (qualities?) are there, not only are they there. For many things are required there. For a natural building shows how many things it requires to be employed (in constructing it).