Alan Pratt in the Internet Encyclopedia of Philosophy tells us:
Grendel, through his mockery of Unferth's heroic postures, has taught Unferth the Dragon's lesson that everything adds up to nothing: "A brief pulsation in the black hole of eternity" (74). But whereas the omniscient, long-lived Dragon can afford to be distant, cool and cynical, Unferth sinks into bitter misery in the face of Life without Meaning. The Dragon practices an absolute materialism, summed up in his advice to Grendel to "seek out gold and sit on it."
Given Pratt's definition of nihilism, it may be that neither The Dragon nor Unferth but Grendel himself is the ultimate embodiment of nihilism. Aaron Hanson seems to disagree. What do you think?