Anna Katschenka, an experienced nurse, takes up a new position at the Am Spiegelgrund clinic in Vienna in 1941. She arrives with high hopes and lofty ideals: The place is a medical institution that treats children with psychiatric problems, and a reform school for wayward boys and girls.

However, she is not long into the job when her boss, Dr. Jekelius, sits her down and shows her a certificate signed by Hitler decreeing the practice of euthanasia. Anna is forbidden from discussing the "acts of mercy" administered within the clinic with anyone outside it. "And you are not to ask any unnecessary questions."

This real-life house of horrors provides the setting for Steve Sem-Sandberg's astounding new novel, "The Chosen Ones." The Swedish author's previous novel, the award-winning "The Emperor of Lies," charted the fading fortunes of Jews in the Polish ghetto of Lodz. "The Chosen Ones," translated by Anna Paterson, sees Sem-Sandberg returning to the Holocaust, focusing on different victims — ill, "imbecile" and perfectly healthy children — and exploring a particularly dark corner where other novelists have feared to tread.

The book alternates between the points of view of Anna and Adrian Ziegler. After bouncing from violent family home to foster care to orphanage, 11-year-old "degenerate" Adrian is finally sent to Am Spiegelgrund. Over the next three years he grows up fast. He and fellow inmates experience all manner of punishments for being "careless" — beatings, dunkings, injections, prolonged solitary confinement and standing straight for 14 hours looking at a picture of the Führer. Inmates who are "hopeless" wither away in cages or straitjackets, until it is time for what one nurse calls "a big clean-out."

The sadistic staff includes Nurse Mutsch, who "hated the children for what they forced her to do to them," and Nurse Mayer, who carries screaming children under her arm "like small animals on their way to slaughter." Anna proclaims her innocence — she is different to her colleagues, she is compassionate and not a member of the Nazi Party — but in time she becomes caught up in the merciless system.

Now and again Sem-Sandberg allows in chinks of light, such as Adrian's two escape attempts and moments of short-lived freedom. But soon we are firmly back in hell. Readers may wonder why they should slog through 500 pages of pain and suffering. The novel is long, and descriptions of children as "unfit for life" are hard to take, but a successful chronicle of grotesque institutionalized abuse during the Third Reich can't cut corners or smooth rough edges.

Sem-Sandberg floors us with a brutal repercussion for a bed-wetter, a boy driven to desperation with stolen scissors and a scabies-encrusted 9-year-old who "potters around the ward all the time, looking for an adult hand to hold." Each devastating, heart-rending instance feels necessary. "The Chosen Ones" may be a tough read, but it is also a powerful and important one.

Malcolm Forbes has written for the Times Literary Supplement, the Economist and the Daily Beast. He lives in Edinburgh, Scotland.

The Chosen Ones
By: Steve Sem-Sandberg, translated from the Swedish by Anna Paterson.
Publisher: Farrar, Straus & Giroux, 560 pages, $28.