The Arthurian Survival. Survival through the Ages, #3

Thrown into the brutal "Great Winter" of Dark Ages Britain, 15-year-old Robert discovers that the reality of the era is far grittier than the legends of Camelot suggest. Amidst a landscape of warring warlords and freezing mud, he must rely on his modern hunting and trapping skills just to stay alive. His high-end, stainless steel hunting knife is mistaken by the superstitious locals for a magical blade akin to Excalibur, turning him into a prime target for every desperate knight seeking power.
While navigating this mythic but deadly world, Robert encounters a "Merlin" who is actually a stranded scientist from the future. Together, they must use Robert's modern survival instincts and the scientist's fading tech to survive the winter and find a way home. In a final, desperate dash through the snow, Robert reaches the shimmering anomaly and wakes up safely back in his own time.