The new video features how Vesemir got initiated as a Witcher and struggled to become one at that. When he became an adult, it was his “profession” to hunt monsters and get paid if he gets the job done.
Vesemir then gets hired by a woman who wants to find out how their villages get killed at night. She even offered a “chaperone” to guide him during his hunt. It turns out she is a witch and are actually kind of at their throats right at the start. After a battle and the witch saves him, he confessed he underestimated him only to tease that did feel only a bit of that.
By the end part of The Witcher: Nightmare of the Wolf trailer, it showed that some king is convinced that monster slaying was staged by the Witchers. It seems this is where humankind started to discriminate against them and started to hunt them down as well. The last part shows Vesemir together with other Witchers fighting against an unknown force, which the video did not show.