The invasion was subtle. Aliens from beyond the stars, they had come to Earth secretly, quietly, over millions of years, until they numbered millions, billions. Then the Earth natives discovered them, fought back, killed them. First a few at a time, then in great numbers. It seemed like the humans were methodically sweeping them off the planet. People of Earth used the invaders' corpses for crafts, incorporating them into vehicles and weapons, works of art and forms of communication. Horror swept through the remaining alien population, and they feared extinction.
Some of the humans called for clemency, to allow the two species to live together in greater harmony. Few listened. The massacre only grew worse, the aliens' ichor sticky on the hands and weapons of their oppressors.
A savior appeared. Having grown among both the aliens and the humans, the alien knew how to appease the natives of their adopted planet. It gave itself up to them, a sacrifice to honor the humans' craft and power. The savior's sacrifice granted the aliens a respite, but the humans demand more sacrifices year after year. The aliens still fear.
And now you know where the tradition of the Christmas tree comes from.