"Know this, then." Said the voice "A lie is a betrayal of our vows. Here, we deal only in truth. If you lie, I will know it. If I hear a lie, I will cut your throat. Do you accept these terms?"
"Yes, I accept them."
"Do you? Understand, I am asking for an oath. Even when I take the knife away from your throat, even when I am dead, even when this knife is rusted and dull and useless, the oath you make by its edge will still be binding. Are you prepared to make an oath?"
"I am prepared,' said Zahariel. 'I will make the oath."
— Descent of the Angels, Warhammer 40,000