This sacrificial ritual was archaic and ancient but easy to set up if you knew what you were doing and you had live enemies on hand. He had no access to runic ink, nor did he have any idea how to make any, so Jon resorted to a mixture of his blood and weirwood sap as a substitute. Blood was a potent magic conductor, and weirwood sap was magically potent. This was not a surprise, considering that weirwood trees practically glowed with magical power, and he even felt a very, very faint connection to them.