unmenschlich
inhuman
Extremely cruel, brutal, or without compassion; violating basic human dignity.
Die Gefangenen wurden unter unmenschlichen Bedingungen gehalten.
The prisoners were held under inhuman conditions.

inhuman / extreme
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inhuman
Extremely cruel, brutal, or without compassion; violating basic human dignity.
Die Gefangenen wurden unter unmenschlichen Bedingungen gehalten.
The prisoners were held under inhuman conditions.

The word consists of the prefix 'un-' and the adjective 'menschlich' (human). The prefix 'un-' negates the base word and here means 'not human' in the sense of cruel or extreme.
Seine Gleichgültigkeit gegenüber dem Leid anderer war einfach unmenschlich.
His indifference to the suffering of others was simply inhuman.
In the meaning 'grausam' (cruel), 'unmenschlich' (inhuman) is often used to describe actions or conditions. Typical combinations are 'unmenschliche Behandlung' (inhuman treatment), 'unmenschliches Verhalten' (inhuman behavior), or 'unmenschliche Lebensbedingungen' (inhuman living conditions).
Die Berichte über die unmenschliche Behandlung der Flüchtlinge schockierten die Welt.
The reports about the inhuman treatment of the refugees shocked the world.
When 'unmenschlich' (extreme) serves as an intensifier, it usually comes before abstract nouns that describe a degree or extent. Pay attention to words like 'Druck' (pressure), 'Anstrengung' (effort), 'Hitze' (heat), or 'Lärm' (noise) to distinguish this meaning from the moral meaning 'grausam' (cruel).
Die Athleten mussten eine unmenschliche Anstrengung leisten, um das Ziel zu erreichen.
The athletes had to make an extreme effort to reach the finish line.
extreme
'extrem' and 'unmenschlich' can both describe something very intense. 'extrem' is more general and can stand neutrally for intensity, difficulty, or special conditions. 'unmenschlich' is stronger and often more emotional. It means either cruel and without compassion or, colloquially, so intense that it is almost unbearable, for example 'unmenschliche Hitze' (inhuman heat) or 'unmenschlicher Druck' (inhuman pressure). If it is only about a very high degree, 'extrem' often fits better. 'unmenschlich' fits when the strain seems very harsh or when someone is treated badly and cruelly.

Die Arbeiter mussten bei unmenschlicher Hitze den ganzen Tag draußen bleiben.
The workers had to stay outside all day in inhuman heat.
Bei der Bewerbung war ich extrem nervös.
I was extremely nervous during the application process.