Black Fog of Morpheus

The Black Fog of Morpheus is a powerful weapon that the Greek God of Dreams; Morpheus use to place the mortals and the other Gods of Olympus to sleep in the video game; God of War: Chains of Olympus.

God of War: Chains of Olympus
After Helios was kidnapped and the sun disappeared, Morpheus decided to take advantage of the situation. He unleashed a black fog that's capable of placing both Mortals and Gods to sleep. Morpheus tried to use the Fog to make him stronger. The fog is also capable of summoning monsters such as the Morpheus' Banshee, Morpheus Shade, and the deadly Morpheus Beast. This fog is proven to be more powerful when it's nighttime, however any kind of source of light like a torch can repel the Black Fog. Morpheus eventually found out that the Gods tasked Kratos to find Helios so the sun can return. So he used the Black Fog to try to hinder Kratos' progress and even summoned some monsters from the fog. Despite his best efforts, Morpheus could prevent Kratos from entering the Realm of Hades with the Black Fog. After Kratos rescued Helios from Persephone and Atlas, the sun returned to the sky. When that happened, the Black Fog disappeared, and Morpheus retreated back into the shadows.

God of War III
Despite Helios getting killed in God of War III, which resulted in the sun disappeared, the Black Fog of Morpheus never appeared. It's likely that Morpheus was either afraid of Zeus and didn't want to be punished again, or Zeus killed him.

Trivia

 * Though it's never stated, it's possible that the Black Fog is Morpheus.