They can be found at Kimbolt Way.
They are weak against Samum, Devil's Puffball, Relict Oil.
Fiends are walking mountains of muscle capped with horned, tooth-filled heads. Like their rarer cousins, bumbakvetches, they live in thick forests, swamps and bogs. When possible they avoid humans, but when not possible, they kill them, and without much difficulty.
Fiends and chorts are among the most dangerous monsters in The Witcher 3. Fiends are huge, hulking creatures, while chorts are essentially smaller relatives - but the latter's no less deadly.
Like cyclopses, fiends will often have the reach to strike back after you've struck a blow, even when you're rolling away. As such, quen is essential once again. When a fiend roars, it's usually about to charge at you, so a sideways roll is a regular necessity.
However, a fiend's favourite trick lies in its ability to hypnotise you. If a fiend suddenly stops, lowers its head a little, and your surroundings start to shake ever so slightly, it's beginning the hypnosis incantation, which, if successful, places you in a dark, vacuous space. In this space, the fiend is free to attack you from out of the darkness, which can make its attack patterns more difficult to predict. Fortunately, the hypnosis wears off after a relatively short time, so it's just a case of waiting it out. That said, your best bet is to interrupt the incantation entirely, by attacking the monster before it finishes.
Meanwhile, chorts sport many of the same attacks as fiends, but their smaller stature means that they're slightly more agile. With this in mind, you'll definitely want to stay at a distance, only attacking when you know the chort isn't about to charge.