Lucius read his death in Regulus' eyes, and casually drew his wand, raising an eyebrow in silent challenge. Regulus could try, if he wished--he wouldn't be the first--though on the whole Lucius thought he likely wasn't that stupid.
"Ah. Of course," Lucius replied, as if he heard such comments every day. "I'm sure He will be thrilled to hear of His mistake."
It was a bluff. Lucius had no intention of telling anyone what he had discovered, not until he decided how to work this new development to his advantage.