The Umbra is a vast well of spiritual energy, similar conceptually to the Silver Flame (and indeed, it was formed in a similar way). Unlike the Silver Flame, the Umbra is a force of shadow and darkness, rather than light.
The Umbra is contained within the Heart of Siberys, in the Ring of Storms in Xen'drik. It was created by the Qabalrin before their civilisation was destroyed (by the Heart of Siberys falling on them), specifically by the Unspoken. The Umbragen found a way to contact the Umbra from Taer Umbra, forming a pact with it to grant them the power they needed to survive in Khyber.