GRNDL is an anomaly amongst machines it eats human flesh. Whether it is just to create a terrifying visage or whether it requires human flesh for sustenance isnt known. GRNDLs presence was only felt when it began to attack the outlying settlements of Midgard, leaving horrifically mangled corpses in its wake. The sheer power and carnage behind the attacks denied the possibility of a serial killer or purely human villain the tracks implied something inhuman and even more horrific.