The sow's head is the gift for the beast, a kind of ritual sacrifice meant to appease the beast and keep it away. But the "gift for the darkness" has other interpretations as already stated. Another is that the sow's meat is also a gift for dark/evil behavior. Later, the boys eat the sow's meat (part of the gift) and devolve further into savage, violent behavior. The word "darkness" appears over 40 times in the novel. And it does have different connotations and meanings in each context. But the scene where they eat the meat is like an evil version of the last supper. Instead of eating the bread of life, they are eating the meat of violence. And in ingesting it, they increase their mentality towards violence; thus, they eat the dark gift (as said above) develop a gift (skill) for darkness.
When Simon approaches the head, he imagines it talking to him:
“Fancy thinking the Beast was something you could hunt and kill!” said the head. For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. “You knew, didn’t you? I’m part of you?
Since the gift for the darkness is also a gift for the beast, there is a clear connection with beast and darkness. They boys think that the beast is some monster or wild animal in the forest. But the sow's head tells Simon that the beast is "part of you." That is to say, each boy (person) has the potential for violence, darkness, evil. This supports the idea that a gift for the beast/darkness is also a "giving into" or "indulging in" that dark potential: Jack and Roger are the most obvious ones who give into and indulge in becoming dark, becoming like beasts.
"Gift for the Darkness" has double meaning in Lord of the Flies. On one hand, the phrase describes the offering left by Jack in chapter eight when he kills the sow and leaves her head on a stick for the beast. That bloody sacrifice might be seen as a gift for the darkness embodied by the beast.
The phrase "gift for the darkness" could also refer to one of the boys in the novel who has a special aptitude for the "darkness" or dark behavior. This description could easily apply to Jack, whose violent tendencies reveal themselves in the bloodthirsty slaughter of the sow. The phrase could also apply to Simon, who has a gift for the darkness, not in violence, but in his ease of wandering through the jungle at night.
Golding's ability to imbue the words, phrases, and symbols of Lord of the Flies with double meaning enriches the text and makes the novel even more challenging.