The rune and ritual performed by Lilith was compared to the parabatai rune and ritual, albeit a more demonic equivalent. If one is wounded or hurt, the other will feel the pain and receive any similar scar, and if one dies, the other would die as well. The rune also makes one, in their case Jace, submissive to the other, practically taking away his free will without him realizing it, giving him the semblance of camaraderie with Sebastian while under the rune's influence. The rune consists of a series of ugly, scrawling, dark red lines.
When Jace Herondale was brought back from the dead by Raziel in 2007, the balance between light and dark was tipped: since a life was returned to the light, a life was owed to the dark. Lilith saw this as an opportunity to bring Sebastian Morgenstern back from the dead as well.
Jace's rebirth made him susceptible to demon influence because he needed to have his protection spells reapplied. Lilith used this vulnerability to give him dreams of him hurting and killing Clary until Jace eventually, accidentally cut her. This caused Jace to feel distraught and desperate enough that when Lilith appeared to him in a dream as his late adoptive brother Max, he believed everything he said about killing the side of him that could hurt Clary.
With blood that Jace himself spilled from a self-inflicted wound, Lilith drew the rune on the left side of his chest. The rune made Jace compliant to Lilith against his will, though he was still conscious of and against his own decisions to follow Lilith. By then, Lilith had also drawn the rune on Sebastian's corpse. Clary tried to break the rune by slashing at it with a knife, and though it seemed to work momentarily and took away Lilith's control over Jace, the rune's true purpose took effect when Sebastian was reborn and Jace became bound to Sebastian.
The only time the rune's power over Jace was broken was when Sister Magdalena cut him with an aegis. The effect temporarily cut the bond and allowed Jace to be himself again and have his own free will, but the rune's effect returned when the wound was healed with an iratze. The rune's power was completely broken during the Battle of the Burren when Clary stabbed Jace with Glorious and burned away all demonic influence in him with heavenly fire.