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Sniffer is the name given to people with the ability to smell violence, the degree to which they can follow it and the length of time it lasts, dependent on the extend of the violence. A simple fight might last a few hours, but a battlefield would last many years (TGH, Ch. 9). Hurin is the only sniffer we have met, but there are at least two more in Shienar alone (TGH, Ch. 9).
Although they normally only smell violence that has occurred, when in the alternate reality, Hurin smells violence that is going to happen (TGH, Ch. 10).
I'm a sniffer, you see. Been one four years this Sunday. I never heard of such a thing before then, but I hear there's a few others like me. It started slow, catching bad smells where nobody else smelled anything, and it grew. Took a whole year before I realized what it was. I could smell violence, the killing and the hurting. Smell where it happened. Smell the trail of those who did it. Every trail's different, so there's no chance of mixing them up. Lord Ingtar heard of it, and took me in his service, to serve the King's justice. (Hurin to Rand, The Great Hunt, Chapter 9).