Alzur's Double Cross is a magic spell used to create huge creatures. The dreaded Viy which destroyed half of Maribor was said to have been created using this spell. It's possible that Fregenal used this spell to create his koshchey in the short story "Droga, z której się nie wraca", either that or the Triangle Within a Triangle.
In The Witcher computer game
The spell is mentioned in the in-game book The Double Cross of Alzur.