Before the current age. There were two highly advanced civilizations. One specialized in biological technology, the other in material technology. The civilization using bio-tech, was at war with the material-tech civilization. They were losing and resorted to forbidden science. The creation of "abominations", has been highly illegal since time immemorial for them. Yet, on the brink of total destruction. They freely violated the old laws to give themselves a chance at survival. Abominations are genetically engineering monstrosities. Their creation is forbidden because the resultant abomination is partially random. No genetic engineer could predict what sort of abomination would arise.
At the start, the abominations were controllable. It halted the enemy, buying them time. Enthused by hope, they endeavored to create even stronger abominations. Which would lead to the creation of the Devourer. They released it on their enemy, happy with how it wiped them out easily. Nearly all by itself, it would dominate a battlefield. Then it could do so all by itself. It would consume an entire planet by itself, leaving no enemies alive, nor life. It ate plants and animals too.
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All this consumption of life increased its intelligence. Turning it from a once mostly mindless beast, into a million year genius towards the end of the war. Its creators never cared to monitor the abomination. They were too excited with victory and never thought it was beyond control for a moment. As the Devourer pretended too well.
When the war was over, the Devourer had no more need for its creators.
"Now to finish my meal..."
It turned on them, ate every member of their species. Thus, the devourer had no more sentient to devour. It began to think, used the left behind knowledge to educate itself. Then it thought to how different beings had different tastes. Although there was one type of being that satisfied him the best.
"Yes, those will be the ones from now on. I no longer have a need for any other kind of food."
It constructed a ship, a combination of technology from both civilizations, and set off into the stars.
The Devourer, would gain more names henceforth.