The supernatural is definitely present in Beowulf, but it's oddly complicated. First, we have the two demons, Grendel and his mother, and the other assorted monsters of the epic – the dragon that Beowulf must fight, the serpentine creatures in the lake, and the sea monsters that Beowulf fought in the past. These are the stock monsters out of an epic fantasy tale. But Beowulf is also deeply conflicted about religion. The poet who composed the poem has a strongly Christian worldview, but also knows that the people he's telling a story about, who are from his distant past, would probably have been pagans. As a result, Beowulf fuses the supernatural together with a religious depiction of evil in a strange and interesting way – the monsters are the Biblical Cain's children.
Instead of a conflict between straightforward good and evil, Beowulf depicts a conflict between demonic fantasy creatures and the power of fate wielded by a Christian God.
Because Beowulf himself seems to be stronger and have more stamina than a normal mortal man, he needs fantastic monsters as his opponents.