To truly prevent anyone from entering, an experimental formula of the Nanite Project was trialled on prisoners of war. After a few less satisfactory results, it was one jackal who, after being dragged screaming and kicking to the operating table, mutated into a host of nanite self-healing rubber, his organic flesh and blood replaced by black latex and his reproductive system drastically altered. Every time he would sense a danger to the Temple, his body would produce an egg to ready himself to convert the intruder into a guard.
It wasn't too long until the drone was parachuted into the lab area, becoming as irradiated and deadly to the touch as his surroundings. Society broke down completely just a few decades afterward, and even the people in hazmat suits holding Geiger counters he could occasionally see in a distance disappeared.
Now, many hundreds of years later, he still guards the entrance of the Temple of the Nucleus. Occasionally small animals venture too close to him, startling him and causing to form another egg inside him, but he has not seen a living being in all the time.
The villagers of the primitive society that has formed on the rremains of the once great civilization tell lore of the huge black six-headed monster that guards the temple which can only be seen from atop the mountains far away, but as anyone who ventures too close immediately gets sick or dies of radiation poisoning, it will still be many more decades until the first living being gets to see the guard drone for real... but will surely receive all of his eggs.
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