Alternate name for Hastur. Although it might appear to be pronounced something like Zastur, if it were intended to be derived from Greek, it would be pronounced as Hastur or, more properly, Khastur (although it would most accurately be Khasteer as the u-sound is not present and the letter written 'u' is an 'i' or 'ee' sound).
In the third edition of the Cthulhu Mythos Encyclopedia (Encyclopedia Cthulhiana), Xastur is said to be a "Female being who may be an avatar of Hastur... said to kill sleeping men" (a little similar to Lilith or Lamia) and "In the teachings of the Critical Mass Theologians, Xastur is the Collective Mind that desires to make contact with the people of Earth."
The name and its Greek origin appear in Justified. The name and Xastur Feminist Net appear in Curse from Xastur Feminist Net.
See The Queen In Yellow, Litany of Xastur and Hastur - The Origin of the Name.