Ulgu

The source of Shadow magic, Ulgu is the Aethyr’s reflection of and reaction to the experience of being lost or confused. This Wind is full of unseen depths, plots, shadows, and illusion. Grey Wizards weave Ulgu into elaborate spells that wreathe them in its mysterious presence. This Lore of Shadows contains spells of darkness, invisibility, shadowy death, and panicked bewilderment.

Ulgu appears to those with witchsight as a thick, impenetrable fog rolling across the earth, invoking a sense of mistrust and confusion in ordinary people who pass through it. Ulgu is drawn to the natural mists and fogs of the mortal world, where it hangs upon the quiet chill of the air, wrapping all in smoky shadows. Ulgu is also drawn to deceptions, mystery, and illusions. Dawn at an ancient stone circle will likely see strands of Ulgu slinking betwixt the monoliths, gradually flowing away as the light of day, and the time of Hyish, grips the land.

Ulgu works subtle changes upon those who call upon its powers—rendering them as mysterious, puzzling, and fleeting as the spells that they call upon. The Grey Order of Wizards use this to their own unknowable ends, acting, hopefully, for the greater good beneath the veil of shadows Ulgu provides. It is hard to remember the face of a Grey Wizard—harder still to track his steps. Those that have tried have returned confused, moon-struck, or occasionally dead.

Reference
Warhammer Fantasy RPG 2nd ED -- Realms of Sorcery (pg. 36)