A Blood Sage is a Fae that can change destiny by writing it with their blood.
Trick is a Blood Sage. When he wore the crown of Blood King, he used his blood powers to write the peace truce and Blood Laws that ended The Great War between Light Fae and Dark Fae. His daughter Aife and granddaughter Bo inherited some of his blood magic.
Anything he writes in his own blood comes to pass, but it comes with a consequence, and a terrible one at that: when Isabeau was killed and Trick wrote the means for ending the war between Light Fae and Dark Fae, it resulted in his own daughter, Aife, becoming a violent rebel determined to disrupt the peace truce. She killed a leader of the Dark Fae clan and was sentenced to death, but was instead imprisoned for many centuries by a cruel Dark King.
In Blood Lines, when Trick wrote to ensure that Aife would not kill Bo when they faced each other in hand-to-hand combat, using his blood power awoke The Garuda. The Garuda was forced into a sort of hibernation when The Blood King used his blood to end The Great War, which deprived it of his source of sustenance: feeding on hatred and anger between the two Fae clans.
The writing power of the Blood Sage only works when he uses it of his own free will. He cannot be forced to change destiny (Lachlan's Gambit).
How the blood power is used
Trick cuts his left hand, lets his blood pour into a vial, and uses it as ink to write runic inscription with a special fountain-style pen.
In Blood Lines, as Trick writes to protect Bo, what seems to be the unfinished Rune for "Mannaz" (the right vertical line is not seen completely written) appears branded on Aife's forehead – her personality suddenly changes, lets go of Bo's hand and falls, thus sacrificing herself for her daughter.
Aife and Bo have inherited a degree of Trick's blood power: if their blood mingles with that of another's, it can thrall the recipient and bind him or her to them.
The symbols written by Trick are similar to Runes. The transliteration of the second symbol (resembling an "R") means ride or journey in the Proto-Norse Runic alphabet. The second symbol (resembling a "W") is closer to an upside-down "e" that means horse. The third symbol (resembling an "A") appears to be an unfinished "m" and means Man.