I couldn't find a picture of Gwilyn, but here's (above) the next best thing: a picture of Chaucer reading to Edward III. Geez, if biting satire was a popular entertainment at court I can easily understand how someone on the receiving end might give up the ghost.
Anyway, here's the exact poem (below) that had the power to kill a man. If you don't want to read it then watch it on the YouTube video (also below):
A SATIRE ON RHYS MEIGEN
by Dafydd ap Gwilyn