MacLeish has exercised his penchant for allegory and moralistic fantasy before, in the delightful The First Book of Eppe. So, if there's an E.T. aroma around this story of an eight-year-old boy who discovers himself born into the line of famous heroes sent into the world to do battle with the Prince of Darkness (on whose account all bad things occur), that's probably just a coincidence. And there is, undeniably, a good deal of charm here. Bentley Ellicott lives with his widowered father in the coastal town of Shorehaven; he has made friends with a young girl, Slally, who speaks unintelligibly (to everyone, that is, except Bentley, who understands her perfectly). Yet Bentley's chief distinction is to have been led-by voices in ""the second air""--to find the secret stone with which every hero of myth (from Hercules to Susanu to Odysseus to Arthur) has used as a shield against Prince Ombra: ""At the moment of its finding by a smooth-lipped one, Prince Ombra is summoned from his kingdom of dark winds and re-enters the world to resume his war with god the nameless who is all the gods ever worshipped by the devout of heart."" Ombra, in fact, lies just off-shore, rippling the black waves at night. And so a half-dozen townspeople are, unbeknownst to them, caught in his snare--doing evil, not loving, being selfish, angry, given to despair . . . until Bentley does final (oddly Freudian) combat with Ombra at the end, in a particularly pleasing scene. Unfortunately, however, this delicate fantasy notion is too often belabored here: the good/evil effects soon become repetitious and predictable. The whimsical language, too, sometimes becomes a precious bore. Still, though things become awfully slow and stretched-out about halfway through, there's much that's sweetly enjoyable here--and the E.T. fever might indeed rub off a bit on this similar serving of child-hero uplift.