In a legend that an introductory note explains was transcribed 800 years ago by ""a monk named Geoffrey,"" the boy Merlin first displays his powers to the wicked King Vortigern. Son of an eldritch lord and a mortal maid, Merlin is an outcast who leads a solitary childhood--until Vortigern chooses him as sacrifice for the site of his Welsh fortification. Suddenly assuming his authority, Merlin reveals the two dragons sleeping beneath the site and predicts Vortigern's imminent death and the coming of Arthur. Service has used the Arthurian legend in some of her novels for young people; here, her first, st picture book is clearly and compactly told, but will be of most interest to children who know something about the hero king. Marshall, whose first illustrations were for Le Guin's Fire and Stone (1989), again uses unusually rich, bright colors and bold strokes and designs in her powerful paintings. Her sturdy, unsentimental figures, vividly expressive faces, and luminous Welsh meadows and hills make this an unusually attractive presentation of a little-known story.