Titus was at rock bottom before he saw Lavinia. Here he compares his grief to an ocean: the extra misery of seeing his daughter mutilated is almost ineffectual because it’s added to so many other miseries, like water to the sea.

“Here ends the Book of the Duchess.” It’s Latin y'all.

And well you know, against nature
It is to live like this;

The narrator’s in an unnatural state, which is bound to lead to unnatural outcomes. Outcomes like trippy dreams about hunting and chess. “And wel ye wite” is an appeal to the reader’s common sense, and is characteristic of Chaucer: we’re always assumed to be at least as intelligent as he is, which is quite flattering considering he’s the father of English poetry.

By what light? On one level this is just a ‘tag’, or pretty much meaningless formula slapped onto the end of a line to make it rhyme (it means ‘by the light of heaven’ or ‘by God’); but the theme of light actually recurs throughout the Book of the Duchesse, and the “lighte” of this line also refers to the light which we later see streaming through a stained glass window onto the narrator’s bed. Chaucer’s tricking us by introducing light in the very first line of the poem, while making it look unimportant.

A proem is an introduction or preface to a text. The Book of the Duchesse is a dream narrative, so the proem tells how the narrator came to have his dream. Chaucer’s just getting warmed up. The dream itself comes later.

In his commentary on the Dream of Scipio, the fifth-century writer Macrobius described something called a visum, which is Latin for ‘vision’ and which refers to hallucinatory, meaningless visions which appear between waking and sleeping. Macrobius was a major source for Chaucer, particularly in the Nun’s Priest’s Tale; critics following Douglas Kelly have identified the “fantasies” here with Macrobius' visions.

Chaucer’s telling us that we’re in for a dream. And it’s gonna be a good one.