Eliezer is a healthy boy in the beginning of Night. At the end he refers to himself as corpse. What are some significant events that caused that?I...

Elie becomes a corpse by the end of the book for many reasons, but also in two facets: physically and emotionally.

Mrs. Schacter and her son in the train would be a good place to begin.  At first the people were compassionate to an old lady who appears to have lost it a little in a crowded train car.  Then, in front of her young son, the crowd begins to beat her senseless each time she calls out about the fire and flames. 

Obviously, starvation and lack of proper food and nutrition in the camps of Auschwitz, Birkenau and Buna reduce a healthy boy's physique to skin and bones.

In addition, the fact that he begins the "journey" as a loving son who is looking out for and protecting his father and ends up trying to lose him in one of the runs so he doesn't have to worry about anyone but himself will reduce a person emotionally from healthy relationships to corpse-like behavior.  The Nazis continually did things to reduce the Jews from humans to "animals".

Another event which dehumanizes Elie and the others is the loss of Juliak's beautiful violin music.  He was killed and his violin was crushed...a symbol for his soul, but also the human connection.

There are many other conflicts which contribute to Elie's metamorphosis--the fight for his shoes, his gold teeth, and all the selections he faced.  Each of these conflicts made him less human and more like the corpse he sees when the liberators appear.