After the apocalypse event, humanity basically had to start from scratch. The systems made to store the information about modern technology were damaged beyond repair, with the new human race having only access to middle age technology and limited, poor quality firearms. In this society which saw itself returning to the use of melee weapons, they had to isolate themselves from the "changed", once humans from the ancient times, now cold monsters devoid of empathy which thrived while the new humans were "grown". What settles these creatures apart is the ability to generate thick, sharp scales akin to those of a pangolin, which they grow on their hands and body to use as swords and bullet-proof Armour. They're mostly composed of keratin
Now, could these "changed" really do that? I've seen concepts of modifying one's arm into a sword, but that would sacrifice flexibility as the wrist joint is rendered useless. Could a creature grow a bladed weapon on their hand, preferably through the association of long scales? The hand is meant to be almost fully covered by the grown structure and be usable again after its discarded by the body.