Vin Diesel had been a Dungeons and Dragons player for many years (he taught Dame Judi Dench to play on the set of The Chronicles of Riddick (2004)). The main character in this movie was partly based on his old Dungeons and Dragons character (Melkor) who was a Witch Hunter. He used one of the third party Dungeons and Dragons books to create the character class, as he loved being a Ranger type, but also like to use a few spells too.
The witch's nest, and surrounding forest, was constructed on an indoor rain set, and was actually burned. It was made mainly from actual tree trunks and branches attached to welded steel frames.
Whenever Kaulder breathes on a window to reveal a symbol, that symbol is an Icelandic magical compass to help the bearer find their way through harsh weather without being injured. The name belonging to such magical symbol is Vegvísir, which is icelandic for wayfinder.
The screenplay for this movie was featured in the 2010 Blacklist; a list of the "most liked" unmade scripts of the year.
The Last Witch Hunter 2 was announced prior to this movie's release, however, after poor box-office performance in the U.S. (the movie did turn a profit in the worldwide market), and disastrous reviews, the sequel is now in doubt.