There were two reasons why the Nyxian Gap developed a fearsome reputation.
The ferocious pirates that called the Nyxian Gap home deterred many adventurers and treasure seekers from entering this mysterious region. If not for their great threat, many outfits would have entered the Gap in order to hunt for alien ruins, traces of rare exotics that normally didn't show up in the galactic rim and other treasures.
The second reason why the Nyxian Gap posed such a great threat was the occurrence of anomalies. The closer to the center of the region, the greater the chance of falling victim to a random anomalous hazard.
Yet that did not mean that the periphery of the Nyxian Gap was safe! The chances that an anomaly might form in the edge of the region was small, but not zero.
Any fleet that entered the region only had to suffer one moment of bad luck to fall victim to a deadly anomaly.