The Shadowhunter's Codex by Cassandra Clare, Joshua Lewis

fight demons rather than Downworlders, has led to a general rejection of firearms as part of the Shadowhunter arsenal.
Finally, it is to the advantage of the Nephilim to have our weapons forged and built by the Iron Sisters as much as possible. Modern gunsmithing involves elaborate industrial machining that our traditional weapons don’t require, and if we were to have the Iron Sisters forge firearms, that would drastically change the Iron Sisters’ need for resources and equipment.
THE TRADITION OF THE PARABATAI
Whither thou goest, I will go;
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 Oath of the Parabatai
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 (See Excerpts from A History of the Nephilim , Appendix A, for more details). 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 and farther from his physical form. David and his followers set down their weapons, exchanging them for a life of mystical contemplation and the pursuit of wisdom.
Before this time, however, Jonathan and David fought side by side as the first parabatai . Tradition tells us that the ritual they performed, where they took of each other’s blood 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 Marks that made him the first Silent Brother. This is a bromance of very heavy-duty proportions.
    You have no idea.
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 to lay down one’s life for the other, to travel where the other travels, and indeed, to be buried in the same place. The Marks 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, Marks made by one parabatai upon another are stronger than other Marks, and there are Marks 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 littered with Shadowhunter-troubadours’ 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 binding can also be cut in the rare occurrence that one of the partners becomes a Downworlder or a mundane. Per above, the bond dissolves naturally