I remember in one it was the ability to feel and feed from the Darkness Beyond Time.
In another it was a shadow dimension (and home to the Golems that Dalton summons) that can be harnessed by certain powerful Shadow-users. The dimension itself is conscious and desires to consume our own universe, so it "uses" Shadow users to try and find itself into our world, possess a suitable host, and consume our world.
In another, similarly enough to the above, the Golems come from some sort of parallel-but-different universe. In that universe they are humanoid, similar to humans (but with pointed ears) and have societies that rival our own. However, the Black Wind allows Shadow users like Magus and Dalton to pull individuals from that parallel world, whereas they are transformed into grotesque beasts upon reaching out world. Magus' attunement to this ability -- the black wind -- is what made him grow pale, have pointed ears, and red eyes, as his overuse of the black wind is slowly transforming him into one of the denizens of the other world. The Black Wind, in that theory, was that it was the ability to pull from this other universe.
