And what rough beast, its hour come round at last, Slouches towards Bethlehem to be born? (lines 21-22)
It’s like the oven bell going off. Ding! "Your Antichrist is ready!" The speaker has a very fatalistic view of the approach of the "rough beast." It hasn’t come because we summoned it, but rather because its hour has arrived. Its appearance is fate, and there’s no way society can avoid it.