" A dark land of ancient plateaus and oceans of night... Night becomes day, and our ancient fathers aid us... We erase that which blights our face, but not that which lies within... The land that rests in twilight will be the place of our greatest triumph and also our greatest shame."
The "dark land" is probably Shakuras, which was bathed in perpetual night for thousands of years. I believe this refers to the activation of the Xel'naga temple on Shakuras, it wiped out all the Zerg on Shakuras, and caused a sunrise
(can be seen in the "Fury of the Xel'naga" cutscene) on the otherwise twilight planet. That explains the Night becoming Day and thw Blight on the face, as well as the ancient fathers (Xel'naga). The blight within and the greatest shame probably refers to the corruption and death of Razagal, she was a revered Dark Templar, and was corrupted from within by her "ally" Kerrigan. Shame is appropriate because Zeratul was forced to kill Razagal himself, it is indeed a shameful tragedy to have to kill your own leader.