"Hm."

Sasuke observed the new seal, not looking up from it to speak. "We sign blood contracts with the animals we summon, and it takes blood to summon them. I had a curse repressed by blood seals, once."

It had hurt. More than getting the initial curse. His shoulder still twinged occasionally, phantom pains that brought him back to Sound. He looked up with a wry smirk. "Didn't work, though."