Darius had the rock faced cleared but it was left empty.  His son Xerxes I had the inscription placed on the face.  The precise location needs to be found. 

It reads:

"A great god is Ahuramazda, the greatest of gods, who created this earth, who created that sky, who created man, who created happiness for man, who made Xerxes king, one king for all, one ruler for all.

I am Xerxes, the great king, the king of kings, king of all kinds of peoples with all kinds of origins, king of this earth great and wide, the son of king Darius, the Achaemenid.
King Xerxes says: King Darius, my father, by the grace of Ahuramazda built much that was good, and he gave orders to dig this niche out, but because he did not make an inscription, I ordered this inscription to be made.

May Ahuramazda and the other gods protect me, my kingdom, and what I have made."

Xerxes I

Xerxes I of Persia (/ˈzɜrksiːz/; Old Persian: Xšaya-ṛšā IPA: [xʃajaːrʃaː] meaning "ruling over heroes";New Persian: خشایارشا ; Greek: Ξέρξης [ksérksɛːs]; Hebrew: אֲחַשְׁוֵרוֹשׁ, Modern Aẖashverosh Tiberian ʼĂḥašwērôš), also known as Xerxes the Great (519–465 BC), was the fourth of the kings of Achaemenid Empire. Xerxes I is more than likely the Persian king identified as Ahasuerus in the biblical book of Esther.^[2][3][4]

• http://en.wikipedia.org/wiki/Xerxes I