Ninja Oath

I am without form, thus without weakness or emotion, I am strong yet I am bendable, I can kill kings without suspicion, I am everything but nothing, I am the darkness that precedes death, I am called by noble causes and fat wallets, I can acquire everything I want through sheer force of will, I am the shadow upon the wall, I am evil yet good, I am ruled by no one, I live by my sword, My face I must hide to protect my soul for when I bear my sword it is brittle, I kill any who see my face, I am ninja