Gawayne and the Green Knight

Arms and the man I sing,—not as of old
The Mantuan bard his mighty verse unrolled,
But in such humbler strains as may beseem
Light changes rung on a fantastic theme.
My tale is ancient, but the sense is new,—
Replete with monstrous fictions, yet half true;—
And, if you'll follow till the story's done,
I promise much instruction, and some fun