Hecate

Hecate is a tribal madwoman who sees herself as a goddess and a prophet, and is percieved as such by a group of fanatics who worship her life/death religion. Mad the goddess may be, but the genetic knowledge she turned into a religion was helping the wasteland. She lives somewhere in the American southwest, in a place called Oroborus. The Daughters of Hecate are her prime servants, spreading Hecate's will to all the tribes. The Vipers, raider exiles from the West, have become Hecate's fanatical army and crush anyone who challenges her. The other tribes fear Hecate's power, and rightly so.

Appearances in games
Hecate did not appear nor was mentioned in any of the published games, but was to appear in the cancelled Fallout 3 project by Black Isle codenamed Van Buren. The player could take her down and free the people from her tyranny (and possibly weaken them in the long run) or let her religion prosper (and build a heartier stock of people that could better survive the rigors of the wasteland).

It is not certain if it will appear in any future Fallout game, but it is known that Bethesda received all Van Buren design documents from Interplay and might re-use some of the background from them.