They're called the Blackhorns. They're a cadre of cultists who worship Nightmare Moon as a goddess. They wear black cloaks with NMM's cutie mark on them, and they always conceal their identity by wearing skull-like masks. However, their top members have slight variations to this design to tell them apart. They're also secretly acquiring resources and ponypower to start a massive war for control of Equestria, while also seeking a return for their dark idol.