Update: CD Projekt Red's press release does shed more light on when Geralt will arrive in the PC version—after the console releases. "The Witcher-inspired content is coming to Monster Hunter: World on PlayStation 4 and Xbox One in early 2019 as a free title update, with the PC release to follow soon."
Doug Cockle reprises his role as Geralt, and the character's combat style will be reflected in-game. The quests aim to blend both games' styles.
Original story: Geralt of Rivia will appear as part of Monster Hunter: World's next big collaboration with The Witcher 3: Wild Hunt. The trailer above shows a little sample of what to expect. Capcom promises that you'll be able to 'step into the shoes' of Geralt, making the most of his legendary monster hunting skills, and participate in a unique quest line.
This comes as part of a free update to the game that will be arriving in early 2019—there's no more specific date than that. The information released doesn't go into specifics on the PC version, so hopefully it'll just land at the same time as the event on consoles. I imagine for fans of both series, though, which include a bunch of PC Gamer's editors, this is a bit of a dream come true. This looks like more than just a throwaway team-up, since Geralt is fully voiced. Hopefully it'll be a pretty elaborate addition to Capcom's massive-selling game.
This isn't the only big news around Monster Hunter: World to break today—a brand new expansion, Iceborne, is on the way too.