Blood - The World of Ice & Fire

Known only as Blood, he and a ratcatcher, Cheese, were responsible for committing one of the Dance of Dragon's most heinous acts; the murder of Prince Jaehaerys. 

Hired by Mysaria, he and Cheese accessed the private residents of the Red Keep, taking Queen Helaena and her children hostage. 

Despite Helaena's plea to kill her youngest son Maelor, Blood decapitated Jaehaerys instead, taking his head as proof of completing their contract.  

Helaena never recovered from the ordeal.