The best approach to answer this question is to use his words. Eliezer is overcome by what he sees as the slaughter in the camps, the fire to which Madame Schachter prophetically allured:
Never shall I forget that night, the first night in camp, that turned my life into one long night seven times sealed.
Never shall I forget that smoke.
Never shall I forget the small faces of the children whose bodies I saw transformed into smoke under a silent sky.
Never shall I forget those flames that consumed my faith for ever.
Never shall I forget the nocturnal silence that deprived me for all eternity of the desire to live.
Never shall I forget those moments that murdered my God and my soul and turned my dreams to ashes.
Never shall I forget those things, even were I condemned to live
as long as God Himself.
This is an effectively painful summary of what Eliezer sees when he enters the camp. He is separated from his mother and sister, the last time he will ever see them alive. He is confronted with the crematorium, the vast plumes of smoke rising that once were human beings. Eliezer sees the death of all that he held dear. Community, solidarity, faith in the divine are all extinguished as the gas and the crematorium rule over everything. He even sees the face of death in the form of Dr. Mengele who operated with the ease of a "musical conductor." The death of children becomes the most painful of all images. For Eliezer, the vast scale of death is an unimaginable one. He hears the Kaddish recited and sees the suffering that was kept silent from him and the world.
Wiesel uses this force of description to convey that which is unable to be conveyed. Rhetorically speaking, how does one convey the death of the camps? Language is insufficient to do so. For example, saying, "There were lots of dead people in the camps," does not effectively convey the burning of the bodies, the dropping of pellets for the gas, the lock of the doors, and the ashes raining down on the living that once were the living. It is with this in mind that Wiesel communicates this through the images that is seen. What Elliezer sees is what constitutes that which he will never forget. In doing so, Wiesel suggests that the way in which one is able to communicate the most horrific of conditions is to describe what one experiences during it. This is the approach he takes in describing what young Eliezer sees around him in "that first night" which he would never forget.