The author's use of descriptive phrases like "that shepherd of evil" and "guardian of crime" in line 323 of the text likely serves to emphasize the malevolence and evil nature of Grendel. These phrases are a form of epic or poetic exaggeration often found in Old English literature, such as in the epic poem "Beowulf," where Grendel is a monstrous character. Renaming or describing characters in such vivid and evocative terms helps create a sense of foreboding and dread, setting the tone for the battle with Grendel and underscoring the heroic qualities of the protagonist, Beowulf. It adds to the epic and mythic quality of the narrative.