He could be overjoyed if he believes the child is his. He would see it as a continuation of his lineage and a sign of their love. He might start making plans for the child's future, like ensuring the best education and training in Camelot.
Well, 'tbate novel' could be a story that involves a complex plot. It may center around a hero's journey, where he or she discovers hidden truths, battles against powerful enemies, and forms alliances. There could be a rich tapestry of relationships, including friendships, romances, and rivalries, all set within an imaginative and often fantastical setting.
Arthur might be initially shocked and confused. He's used to Merlin being in control. But then he might find it quite hilarious and start teasing Merlin about it.