This was submitted for a scifi anthology, but as it was accepted I'll post it here. Dark and NSFW.
A creature born of energy from the stars, unique and unknown to other worlds. It snaked through the galaxy, visiting inhabited planets, stopping only to replenish basic needs. It was alone, but didn’t know any different, until a chance encounter on a primitive planet.
It landed on the planet hungry and desperate to feed. As it hunted its food through a forest, it discovered two beings in a sunlit-dappled glade. Unaware of their voyeur, they shared breath as if the oxygen had been sucked from their world. It watched, partially hidden by a large tree, fascinated by this ritual. Their hands traced patterns over bare skin, gripped and scratched, marring their pale, lightly furred skin. One penetrated the other from behind, with a shaft engorged by blood. It could smell the beings from its hiding place; smell the sweat, and the blood pumping through the arteries of their bodies. Their movements became harder, less rhythmic, and their noises as erratic. The shaft of the one who was penetrated leaked a fluid, to be shared and smeared over their skin. More scents to add to the cocktail which teased the senses of the alien creature and left it wanting.
At that point it knew it could take more than food from each planet. It took the corporeal form of this planet and approached a being. It was renewed and moved on to the next giver of life and sex and food.
It stretched languorously as the power surged through its corporeal form, satiated for the first time in eons. As it played with each one, it learned to draw out their pleasure, taste them, and drain them, until nothing was left.
A world of bone and blood consumed, leaving nothing but bare husk behind.