Being shadow kissed means that one is mentally connected to a spirit user. Usually the mental attachment is referred to as a "bond."
When the person dies, if a spirit user heals them to full health right after, it pulls their soul back from the Land of the Dead. The person's soul is forever branded by the land of the dead. If the shadow-kissed person kills, they are plagued with ghosts from the other world constantly unless the person places a mental barrier around themselves or is inside a Ward. They can be freed from the spirit bond if they are hurt in a life threatening way and survive with no spirit induced healing at all.
When the spirit user uses spirit, the negative effects from that build up in the spirit user and then pushed into the shadow kissed or, in some situations, even pulled into the shadow-kissed by the shadow-kissed to save the spirit user from doing bad things to themselves or someone else. The shadow-kissed can eventually go mad if they constantly take the darkness from the spirit user they're bonded with (as the case is with St. Vladimir and Anna).
A spirit user may heal a Strigoi by impaling the Strigoi in the heart with a silver stake that is infused with fire, air, earth, water, and spirit. When the spirit user does this, no bond is formed. The reason for this is because when a Strigoi is made, the soul does not go to the Land of the Dead. The soul just hangs around on Earth, waiting to be reunited with its body or to be forced into the land of the dead. So, the healed Strigoi does not become shadow-kissed and instead is restored to the person they were before becoming Strigoi.
Known Shadow-Kissed PeopleEdit
From book one, Vampire Academy, we know that Rose is shadow kissed. The bond saves Lissa's life a few times through out the series and even lets Rose check in on Lissa while Rose is in Siberia, Russia in Blood Promise.
In Last Sacrifice when Natasha Ozera shoots Rose in the chest, Lissa couldn't heal her in time. Rose pulled herself back from death and the bond was broken with Lissa because Rose was finally strong enough to be without it.
St. Vladimir, the saint whom the Academy is named after, is a spirit user. Back then, people knew about this strange and rare fifth element.
Avery Lazar first discovered her spirit powers when she saw Simon die in battle. She revived Simon with no knowledge of how she did it. Simon was Avery's first bond mate.
It is believed that Avery killed her brother Reed to then try to bring him back to life so she could have two shadow kissed people attached to her and so she could use more spirit without all the nasty side affects.
In between Last Sacrifice and Bloodlines, Jill was killed by the rogue Moroi at a dinner party. The rogue Moroi made the attack look like it was for Queen Vasilisa Dragomir, but they went for Jill. Adrian, horrified at seeing a knife through Jill's chest, without a second thought brought Jill back to life. When the two move to Palm Springs in Bloodlines, things affect their bond like no one thought could ever happen.