Answer:
Legend
Explanation:
In English language, a legend can be defined as a traditional narrative or story that is written about a particular person or past events and it's likely to either be a myth or historical facts. Thus, a legend is sometimes an unrealistic story or narration that describe a particular person or an extraordinary event that occured in the past. Also, it's a story about a particular figure, past human events or actions whose origin is unknown, unproven and undocumented in real history
Additionally, the word "legend" was derived from the Latin word "legendus" which simply means something which should be read.
Hence, the most appropriate word to complete the sentence is a legend because there are no proven and documented or written records about how Bolster the Giant terrorised the people of Cornwall. Thus, the complete mythical sentence is then written as;
"According to legend, Bolster the Giant terrorised the people of Cornwall."
