The Perseus Veil

Mass Effect 2 & Mass Effect 3
As vast in natural beauty as it is in threat, the purple and gold nebula called the Perseus Veil forms the natural border between geth space and the Terminus Systems.

The Veil's total opacity prevents Council intelligence from surveying geth activity. Theoretically, the geth could be preparing a devastating attack against which the Council could be defenseless, or the geth could have died out, so that the defense budget against them could be gaining the Alliance nothing but economic ruination.

Despite fears of geth, prospectors do occasionally mount savage ventures inside the Veil; one ended in tragedy. Using technomental domination, the geth drew the team into the Veil before aiming them back as husks at the organic society that produced them.

A leaked classified Spectre report claims that the dreadnought Sovereign, commanded by ex-Spectre Saren Arterius and crewed by geth, hid near the Veil before initiating the 2183 Citadel attack.