Warrens & Holds
The magical paths and their revelations through the series
Understanding the Magic System
Elder Warrens: The ancient sources of magic, tied to Elder races (Tiste, Jaghut, T'lan Imass)
Paths (Warrens): Mortal reflections of Elder Warrens, accessible to human mages through the Deck of Dragons
Holds: Pre-Warren magic system used in Lether, accessed through the Tiles
K'rul and the Creation of Warrens
As revealed in Memories of Ice, K'rul the Elder God created the Warrens by spilling his own blood, allowing mortals to access magical power. Before this, only the Elder Warrens existed, tied to specific races. The mortal Warrens are essentially paths through K'rul's blood, which is why he grows weaker as fewer worship him - the Warrens themselves are his body.
The Holds of Lether represent an older system, predating K'rul's sacrifice. They are accessed through the Tiles rather than the Deck of Dragons, and are ruled by the Errant rather than tied to K'rul.