The Continent
The world is inspired by The Witcher series by Andrej Sapkowski (and the CDPR games), and the creatures and locations presented in the handbooks for D&D 5e by Wizards of The Coast.
In the Middle Ages of Europe, a calamity occurs. The human world joins with another world, full of magic and unnatural entities, through a phenomenon known as the Conjunction of the Spheres. Though the Conjunction is brief, magic 'leaks' through and permeates existence. And along with it, dangerous monsters cross over and proliferate the world, making it their home. Some are docile. But most are violent and predatory.
As a defence against these creatures, humanity created a creed of warriors, granted extreme physical capabilities (and ostensibly stripped of emotions) through mutations and armed with weapons training and magic. As long as there are things that go 'bump' in the dark... a Witcher will 'bump' back.
Witchers are trained in different Witcher schools across the land. They travel The Continent in search of contracts, in which they offer their monster-slaying expertise for money (importantly, a Witcher never works for free). They are regarded with caution and even fear in the eyes of common folk, for their unnatural appearance, gruff manners and the various rumours painting them as devils. Although, a few witchers have become well-known and even popular among learned citizens as the subjects of bards' ballads. The most notable of them is the White Wolf, Geralt of Rivia.
Currently, Witcher populations are on the decline as no school trains new Witchers and monsters become increasingly sparse due to their effectiveness.
Aside from Witchers, magic and alchemy are prevalent in these lands, and the standard professions of the Middle Ages still exist. The other D&D 5e classes and races are available for play.