Take a look at the sentence again:
For a sentence to be fused, you must have two complete sentences run together with no punctuation. In the example above, a comma joins the two sentences. Mike loves to play computer games, especially Tomb Raider is one sentence. He imagines ... begins the second sentence. Thus, the error is a comma splice.
Remember that both comma splices and fused sentences are major errors. They make your reader think that you cannot write a correct sentence. Because the sentence is the most basic building block of a piece of writing, comma splices and fused sentences make you look like an amateur!
To fix the problem above, you could put a period after Raider and begin he with a capital H.