As you know, there are two types of relative clause:
The woman who lives next door is a doctor.
In this example,the relative clause tells us which person or thing (or what kind of person or thing ) the speaker means. We don't use a comma with this clause. Also, we can use that.
My brother Ben, who lives in Hong Kong, is an architect.
In this example, the relative clause does not tell you which person or thing the speaker means. We already know which thing or person is meant. The relative clause in this sentence gives us extra information about the person or thing. We use a comma with this clause. Also, we cannot use that.
Now, I came across a sentence that doesn't follow these rules:
Sing, O goddess, the anger of Achilles son of Peleus, that brought countless ills upon the Achaeans. Many a brave soul did it send hurrying down to Hades, and many a hero did it yield a prey to dogs and vultures, for so were the counsels of Jove fulfilled from the day on which the son of Atreus, king of men, and great Achilles, first fell out with one another.
(It comes from the Illiad translated by Samuel Butler, and I thought that grammar rules haven't changed much since then.)
Please explain this contradiction, as far as you are able.