During World War II De-Nur spent two years as a prisoner in Auschwitz. In 1945, he moved to British-mandate Palestine (later Israel) and became a writer-historian survivor who wrote several works in Hebrew, which stemmed from his experience in the camp, under the identity he had been given by the guards at Auschwitz: Ka-Tzetnik 135633 (sometimes listed as "K. Tzetnik"). Ka-Tzetnik means "Concentration Camper" in Yiddish; 135633 was De-Nur's concentration camp number. He also used the name Karl Zetinski (or Karol Cetinsky) as a refugee, hence the confusion over his 'real' name when his works were first published .
De-Nur's writings were an attempt to chronicle his time at Auschwitz. He wrote using the name Ka-Tzetnik 135633 (which he denied was a pseudonym or pen name but was what his time in Auschwitz had made him) for some time before his civic identity was revealed at the trial of Nazi leader Adolf Eichmann at court session 68 of the trial on 7 June 1961 . After an opening statement in which De-Nur described Auschwitz as the "planet of the ashes", but before he was able to answer the general questions about Auschwitz that the prosecuting Attorney-General had prepared for him, De-Nur fainted and was subsequently unable to resume his testimony.
Among his most famous works was The House of Dolls (1955) , which described the "Joy Division", a Nazi system that kept Jewish women as sex slaves in concentration camps. He suggests that the subject of the book was his younger sister, who did not survive the Holocaust. In his book Piepel, about the Nazis' sexual abuse of young boys, he suggests the subject of this book was his younger brother, who also died in a concentration camp While De-Nur's books are still a part of the high-school curriculum, modern Israeli critics like Na'ama Shik consider The House of Dolls pornographic fiction, not least because sexual relations with Jews were strictly forbidden to all Aryan citizens of Nazi Germany. Its publication led the way to the success of the Nazi exploitation genre of popular literature, known in Israel as Stalags series of pornographic novels that were produced by Ezra Narkis.
In 1976, because of recurring nightmares and depression, he subjected himself to a form of psychedelic psychotherapy from Dr. Jan Bastiaans that included the use of LSD; the visions experienced during this therapy became the basis for his book, Shivitti