A person or thing that gruels; something that exhausts, tests severely, or wears someone down.
From 'gruel' (as a verb, meaning to exhaust) plus the agent suffix '-er', creating a noun for something that causes grueling exhaustion.
A 'grueler' is someone or something mercilessly demanding—notice how English lets us name not just the victim of exhaustion but the thing causing it, reflecting how we identify and talk about sources of suffering!
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