The Question and Answer section for Poems of W.B. Yeats: The Rose is a great resource to ask questions, find answers, and discuss the novel.
That would be the force of nature.
Man is full of violent hubris that seeks to fulfill his desires at the cost of nature. Man does not understand that his greed and conceit will lead to his own undoing.
The poem indicates that if man does not change his ways of greed and hubris, the world will be cast into darkness. The natural world will unleash its fury on mankind.