The second stanza of Dylan Thomas's villanelle "Do Not Go Gentle into That Good Night" reads as follows:
Though wise men at their end know dark is right,
Because their words had forked no lightning they
Do not go gentle into that good night.
The poet's first use of the phrase "Do not go gentle into that good night" at the beginning of the previous stanza is imperative. Here, however, he is explaining why this particular category of people choose not to die meekly or quietly. One might expect that wise men would be the very people to accept death with equanimity, particularly since they acknowledge that "dark is right." However, they cannot die in peace, because their words "had forked no lightning."
The image here is a striking one. Forked lightning is dramatic and leaves a lasting effect on the landscape. This is what these wise men, for all their wisdom, have been unable to do. Their words of wisdom should have had the dramatic and lasting effect of bolts of forked lightning. The fact that they have not had this effect means that, at the point of death, they feel their work on earth is not yet done. This is why they struggle against death, attempting to produce words of forked lightning which will change the world and secure their immortality, intellectually if not physically, before the darkness overwhelms them.