Writ of Union

The Writ of Union is an oath of service to the Covenant, written by the San 'Shyuum and Sangheili who created the Covenant, apparently the Covenant High Council of Concordance at the end of their war. In the old Covenant tradition, when an Elite became a warrior, he recited the oath. This writ was apparently authored and first documented in the first Age of Reconciliation (852 B.C.E.).

Overview
Part of the defining union between the San 'Shyuum and the Sangheili was the division of labor it created: the San 'Shyuum would serve as the religious leaders and scientists, studying Forerunner technology and seeking how to use the Halo Array to initiate the Great Journey, which would propel the faithful to join the Forerunners in a god-like state of transcendence. In return for sharing this knowledge, the Sangheili would serve as the San 'Shyuum's protectors and warriors, using weapons and technology developed by the San 'Shyuum, and having total control over all security matters. As the Covenant expanded and incorporated other races, they were added to Writ of Union, serving as soldiers under the Sangheili.

The Oath
The full content is unknown, but here are some excerpts:

Beginning
The beginning of the Writ of Union is as follows:

So full of hate were our eyes
 * That none of us could see

Our war would yield countless dead
 * But never victory

So let us cast arms aside
 * And like discard our wrath

Thou, in faith, will keep us safe
 * Whilst we find the path

The Arbiter (Level)
On the Halo 2 level, The Arbiter, in the opening cut scene, SpecOps Elites recite this before they land :

Rtas 'Vadumee: "When we join the Covenant, we took an Oath!"

SpecOps Elites: "According to our station! All without exception!"

Rtas 'Vadumee: "On the blood of our fathers... on the blood of our sons, we swore to uphold the Covenant!"

SpecOps Elites: "Even to our dying breath!"

Rtas 'Vadumee: "Those who would break this oath are Heretics... Worthy of neither pity, nor mercy! Even now, They use our Lord's creations to broadcast their lies!"

SpecOps Elites: "We shall grind them into dust!" {''The subtitles add to this line with "...scrape them as excrement from our boots." - see trivia.''}

Rtas 'Vadumee: "And continue our march to glorious salvation!"

Early Halo 2 script
On and on shall old war go,

Without respite my blood will flow

O’er your eyes ‘til they cannot see

The impossibility of victory.

Trivia

 * High-ranked Covenant Unggoy Deacons, such as Dadab, were required to memorize the entire Writ of Union, word for word.
 * During gameplay, the extra line "scrape them as excrement from our boots" in the subtitles is added to the oath by the Special Operations Elites after "We shall grind them into dust," however, it remains unspoken. The reason for this omission is unknown, although it is most likely because Elites do not wear boots given their split toed and hoof-shaped feet.
 * The Writ of Union is written on the inside of the Halo 2: Original Soundtrack Volume One.