The Massacre was one of the most horrible and deadliest events in Mekahuman history. It was the deaths of most of the Mekahumans after a coordinated attacked by the Opposites and The Spirit. It happened in 1739, and only three Mekahumans survived, along with 23 Descendants and 27 Grands.
After the Massacre the Mekahumans, Descendants, and Grands went into hiding, in an attempt to preserve the existence of their species. They still remain very scattered, with little to no contact between each other when Gift of Power begins.
The Massacre allowed for the Opposites to gain more power in the non-human world, as even through an equally large amount of Opposites had been killed during the Massacre, the Mekahumans were too afraid to come out of hiding.