A Shrouding Spell is a spell which create a magical, protective barrier over something. The shroud requires three Wizards to join together and say Raay-Sha-Da. The shroud hides wizards when they cast spells so the Nekross can't find them and then extract their magic. Exceptions include Grim Magic and a Hobbledehoy's Magic.
Jackson Hawke's Grim Magic was so powerful, the Nekross could detect it despite the shroud.
The magical shroud was damaged when Jathro and the Nekross identified the Zeyton particle which was able to penetrate the shroud. Moon and Benny find out that a Mirror of Enchantment will allow Randal Moon to mend the shroud. Although Varg smashed the Mirror, Moon, using science and magic, created two doubles of him and together mended the shroud and made it stronger than before using Nekross tech.