Apostles were attacking true animean humans that are infected by vampire bites in order to prevent an upcoming vampire attack.
An Apostle is a sapient species that can look like a former human who lives within the Berserk universe, and that in a moment of ultimate despair has used a Behelit to sacrifice their loved ones or dream to transcend their humanity and become a demon and a servant of the God Hand. Each Apostle receives a demonic body capable of transformation. Every 216 years, they gather to the place chosen for the eclipse ceremony, where they can feed off of the sacrificed. Some Apostles have been known to defy the God Hand's will, notably Wyald and Emperor Ganishka, either because of their unruly nature or their hunger for power. While the one "commandment" imposed on all Apostles is "Do as thou wilt," the Apostles' doings do not receive protection through that commandment, enabling Apostles who are loyal to the God Hand to exact retribution on rebellious Apostles.
Nowadays, they are now free from the God Hand and Griffith's service, and like all demon species on Earth, they are also actually peaceful and gentle, unlike how the characters of Berserk once thought. They are omnivores, feeding mainly on the same food as humans. They come in many different shapes and sizes depending on a subspecies. They are not effected by religious items, unlike in mythologies. They aren't evil and only feed on live animals in case of food shortages. Also, instead of turning humans into Apostles, they can breed and have babies of their own kind, but can also breed and have babies of mixed kinds if an Apostle is married to a member of the other sapient species. Also, they don't need the Satanic God Hand, and can now instead worship any other religions of what they want (like Christianity, Islam, Buddhism, etc.). Also, they still have to gather to the place chosen for the eclipse ceremony, but can feed off of the sacrificed Apostle instead of any other sapient species, just like xenomorphs, where those creatures can kill off other xenomorphs with facehuggers, and can turn into chestbursters that would sprout out of the xenomorphs' chests. Also, they can obey the rules of the religion and the government instead of the "Do as thou wilt" rule, which it would be malevolent for everyone. Also, they can receive protection of what they want.