In Night what was Rabbi Eliahou's effect on prisoners?
This is a good question. Rabbi Eliahu is only mentioned twice in the book. So, he did not make an impact on the prisoners. We can assume that since he was a religious figure, some prisoner's found comfort in his presence. Others probably did not care one way or another. However, what we can say is that the Rabbi made an impression on Elie for a specific reason.
The first time the rabbi is mentioned, the rabbi is talking with Elie. He lost his son, and so he was asking Elie if he by chance saw him. Elie said, "no."
Later Elie remembered something. He realized that he did see the rabbi's son. The son ran ahead and even peeked back to see his father. He continued to run, because he did not want to be held back by his aging and weakening father. The situation was so bad that sons betrayed fathers. Self-presevation ruled.
When Elie realized this, he prayed that he would never become like the rabbi's son.
"Oh God, Master of the Universe, give me the strength never to do what Rabbi Eliahu's son has done."
When the rabbi is mentioned again, Elie begrudgingly took care of his father. When he realized this, he knew that he experienced the heart of the rabbi's son.
I gave him what was left of my soup. But my heart was heavy. I was aware that I was doing it grudgingly.
Just like Rabbi Eliahu's son, I had not passed the test.
Rabbi Eliahou allows for prisoners, especially Eliezer, to think and care more about their fathers. Rabbi Eliahou has been a good man who was admired by all. He and his son had stayed together in the camps for a long time, until one day he runs away from his father who started falling behind. Rabbi Eliahou thus starts to look for his son, but never finds him in the end. Eliezer later prays to god so that he would have strength to avoid abandoning his father as Rabbi Eliahou's son did.