The Blinder's Knife, also known as Hellfang or Marrow Sucker, is a mysterious and powerful weapon that is able to take away a drafter's abilities permanently, either wholely or in part.
Katalina Delauria kept the weapon for 15 years, passing it on to Kip when she was dying from injuries she suffered in the Massacre Of Rekton. Rask Garadul then took the knife from Kip during the Massacre. The Colour Prince, who was ostensibly allied with Rask, then gave the knife to Zymun, along with a mission to assassinate the Prism, Gavin Guile, secretly Dazen Guile, and therefore Zymun's uncle. Zymun failed due to the intervention of Kip, and only wounded Gavin in the kidney. However, this was enough for Hellfang to start its work, and soon afterward, Gavin lost the ability to see or draft Blue. One by one, he lost his colours, until he was rendered colourblind and a non-drafter.
It appears that the knife does not always take away drafting entirely. Andross Guile had been a hair's breadth from breaking the halo for years until he was injured with the knife, and as a result of his exposure, was rejuvenated by Hellfang taking away some of the built-up Luxin in his body, and not only regained his vitality, but was able to safely draft again (for the foreseeable future, until his halo built up again.)