Entreat me not to leave thee,

Or Return from following after thee-for 

whither thou goest, I will go, 

And where thou logest, I will lodge, 

 Thy people shall be my people, 

and they God my God.

Where thou diest, will I die,

and there will I be buried. 

The Angel do so to me, and more also, 

if aught but death part thee and me. 

The tradition of the Parabatai goes back to the beginnings of the Shadowhunters; the first Parabatai were Jonathan Shadowhunter himself and his companion, David. They in turn were inspired by their coincident namesakes, from the biblical tale of Jonathan and David: 

"And it came to pass..., that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul...Then Jonathan and David made a covenant, because he loved him as his own soul." -1 Samuel 18:1-3 

Out of that tradition Jonathan Shadowhunter created the Parabatai, and codified the ceremony into law. 

David the Silent was not at first a Silent Brother. At first there were no Silent Brothers; earliest Nephilim hoped that their more difficult and mystical roles could be integrated into their warrior selves. Only as time passed did it become clear that the work of David would take him ever toward the angelic and farther from his physical form. David and his followers set down their weapons axchanigng them for a life of mysitcal contemplation and the pursuit of wisdom. 

Before this time however, Jonathan and David fougt side by side as the first Parabatai. Tradition tells us that the ritual they performed, where they took of each other's blkood and spoke the words of the oath and inscribed the Runes of binding upon each other, was the second-to-last time that David was known to shed human tears. The last time was the moment when the Parabatai bond was broken, as David took the Runes that made him the first Silent Brother. 

Today Parabatai must be bonded in childhood; that is before either has turned eighteen years old. They are not merely warriors who fight together; the oaths that newly made Parabatai take in front of the Council include vows that lay down one's life for the other, to travel where the other travels, and indeed, to be buried in the same place. The Runes of Parabatai are then put upon them,  which enable them to draw on each other's strength in battle. They are able to sense each other's life force; Shadowhunters who have lost their Parabatai describe being able to feel the life leave their partner. In addition, Runes made by one Parabatai upon another are stronger than other Runes and they are Runes that only Parabatai can use, because  they draw on the partners' doubled strength. 

The only bond forbidden to the Parabatai is the romantic bond. These bonded pairs must maintain the dignity of their warrior bond and must not allow it to transform into the earthly love we call Eros. The late Middle Ages were were littered with Shadowhunter-troubadour's songs of the forbidden love of Parabatai pairs and the tragedies that befell them. The warnings are not merely of heartache  and betrayal but of magical disaster, impossible to prevent when Parabatai become romantically linked. 

Like the marriage bond, the Parabatai bond is broken, normally only by the death of one of the members of the partnership. The binging can also be cut in the rare occurance that one of the partners becomes a Downworlder or a Mundane. Like previsously stated the bond dissovles naturally if one of the partners becomes a Silent Brother or Iron Sister: the Runes of transformation that new oblates take are among the most powerful that exist and overwelm and dissovle more ordinary warrior Runes. 

A Shadowhunter may choose only ONE Parabatai in his or her lifetime and cannot perfom the ritual more than once. Most Shadowhunters never have any Parabatai at all; if you newly blessed Nephilim find yourself with one, consider it a great blessing.