Witchcraft is the in game (IG) term for players who have multiple character accounts, i.e. multiples or clones. In the strictest interpretation, players are allowed only one single character throughout the game.
"The users are allowed to create only one account on the website of the Renaissances Kingdoms, and only they can use it." - Renaissance Kingdoms - Terms and Conditions; The Game
A looser interpretation of anti-witchcraft laws is based on the idea that multiples that don't benefits each other are permissible. Players who follow the looser interpretation are expected to keep their multiples in different counties, and preferably, in different countries.
This type of witchcraft has no relation to the Pagan_Religion of the same name.