I believe John Gardner gave the book this title because it is short and describes the same character. The word Grendel itself sounds like a monsters name and makes you picture it in your head what Grendel actually looks like. When you think about monsters you usually think about heroes also, so John Gardner wrote the story to make you think about Grendel as the monster, so in your head you can think about what the hero has to be capable of in order to slay or defeat the monster.
The story is set in Grendels twelfth year of attacks. Grendel encounters a ram, the ram doesnt pay attention to Grendel and it upsets him. So Grendel throws rocks at the ram and stomps the ground, but the ram just stares at him, this reminds Grendel that Spring is almost there. Spring has begun and Grendel is angry and cant resist it any longer, so he heads for the mead hall where he kills Hrothgars men and takes the bodies into the woods, where he eats them. This is the twelfth year of Grendels war. Then the story takes a turn and goes back
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