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arabatai is a pair of Nephilim warriors who fight together and are closer than siblings, regardless of their gender. They are partners who would lay down their life for the other. Being ones' parabatai is not merely a matter of promising to guard each other. The pair must stand before the Council and swear an oath to lay down their lives for each other.
Not all Shadowhunters are required to have parabatai; it is actually less common to have them. A Shadowhunter has only eighteen years to find and choose a parabatai; once older than that, the ritual is no longer open to the Shadowhunter.
Shadowhunters are also forbidden from falling in love with their parabatai. Most Shadowhunters don't know the reason, but it apparently stems from a dark Clave secret. This is because the pairs must maintain the dignity of their warrior bond. The Shadowhunter-troubadours' in the late Middle Ages made several songs depicting the forbidden love of parabatai pairs and the tragedies, both of heartache and magical disaster, that befell them when they become romantically linked.
The Oath
Entreat me not to leave thee,
Or return from following after thee?for whither thou goest, I will go,
And where thou lodgest, I will lodge.
Thy people shall be my people, and thy 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.
