Manannan is the chief antagonist of the computer game King's Quest III: To Heir is Human, a wicked wizard who held the youth known as Gwydion prisoner for many years and treated him as a slave he is a cold-hearted figure with little regard for anyone save himself.
Manannan is also shown as being cruel, turning Gwydion into a snail as punishment when he fails to do a chore - yet restoring him to normal afterwards.
It is revealed later in the game that Manannan also kidnapped Gwydion, who was in reality a prince and heir to the throne called Alexander, after many adventures Gwydion returns and ends Manannan's evil reign via transforming the once proud wizard into a cat.
In King's Quest V: Absence Makes the Heart go Yonder, Manannan's brother Mordack kidnaps Alexander again along with the rest of his family and demands him to restore Manannan to human form. Alexander's father, King Graham, has to save them. Inside Mordack's castle, Manannan is encountered as a cat and will alert Mordack to Graham's presence when talked to (like all animals in the game, he is capable of speech). He is defeated by distracting him with a fish and then trapping him in a bag. It is unknown what happened to Manannan after this.