Lestat did not laugh, but mentally, he was cackling. A promise like that from a vampire was generally worth very little. While Lestat did have respect for Eric, he himself could not even promise not to harm Honour. One day, he was sure, it would happen. Especially if he cared for her. But a promise not to Glamour, that he could perhaps believe.
And he himself would hold Eric accountable if that promise was broken.
He smiled when she mentioned that he was part of the night.
And as for how he managed to be in the sun...
"Someone gave me a gift," he said again. "That is really all I can tell you. It is what is allowing this, and what will allow it for a few more days."
There may be other benefits to this, but those would be the large ones, he knew. And it had come with a price. In some way, he was certain he'd made Eric more indestructible.
He gave her another warm smile. "It's mostly magic, anyway. What makes beings like me different. Magic explained never sounds as good as the reality."