Literally, the sign that comes from the world of grownups is a dead paratrooper who falls from the sky and gets tangled in the trees at the top of the island.
The twins Eric and Sam see it and are frightened. At this point, it takes on figurative meanings. To them it is the dreaded "beast." They describe it as follows:
"It was furry. There was something moving behind its head—wings. The beast moved too—" "That was awful. It kind of sat up—" "The fire was bright—" "We'd just made it up—" "—more sticks on—" "There were eyes—" "Teeth—" "Claws—"
This fantastic notion of the dead paratrooper as a beast becomes the "truth" the boys now accept. The paratrooper morphs into the symbol of what is threatening, feared, and to be destroyed. The boys project onto it their own fears and make it larger than life, a monster rather than a corpse. Primitive emotions swamp reason and lead to bad outcomes when Simon is mistaken for the beast.