This is a good question. I assume that you are talking about, the book, The Sibyl by Pär Lagerkvist. This is a dark work of philosophy and theology that explores the meaning of life among other things. Ahasuerus and the Sibyl have a very long conversation about their lives and how they got to where they are currently.
Ahasuerus basically states that he had a great life, but one day he chased someone from leaning on his house bearing a cross, lest he be contaminated. But it was this act of chasing the man from leaning on his house that has brought down a curse on himself. The curse was to wander the earth without ever finding rest.
These words perplexed him so much. So, he is looking for an answer. Therefore, his question is about his life and fate. What will happen to him in the future? He ask the Sibyl and she has no answer. She only says that his fate is intertwined with that of the gods.