Written in 1930, published in 1945 in The Welsh Review.
I had completely forgotten the existence of this poem - although it is listed in the bibliographies, and despite that I had (but some decades ago) read a description and critique in Paul Kocher's Master of Middle Earth - only in re-reading that book did I remember, and found an online (scanned, very slightly inaccurate) copy:
NOTE: I have placed a complete copy of the poem in the blog post above-
just in case the above web pages become unavailable or are deleted.
Aotrou and Itroun is a vivid, ballad-like and Christian tale, set in the Middle Ages in Brittany, concerning temptation, black magic, repentance, retribution.
As might be expected from Tolkien - the end is sad, but ultimately hopeful.
It is a very good poem, I would judge (but after only a single reading) it is perhaps Tolkien's best long poem - and well worth reading both for its style and the content.