The Mark of Transference was a mystical skin marking that imprinted on Lana Lang's back through magic, as part of an enchantment by Isobel Thoreaux to preserve her soul.
In Crusade Lana received the mark when she visited Isobel's tomb in Paris. Lana woke up in her bedroom immediately afterward not remembering the last twelve hours. When she purchased the spell book, the mark on her skin combined with the identical mark in the book, which was stained with Isobel's blood. This allowed Isobel's spirit to possess Lana. Once Lana killed Genevieve Teague, the tattoo vanished, the spell was broken and Lana was freed from being controlled by the Countess ever again.