The disciples of Tatian, who, after the death of Justin Martyr, “formed a new scheme of religion; for he advanced the notion of certain invisible aeons, branded marriage with the name of fornication, and denied the salvation of Adam.” (Irenaeus. Adv. Hereses (ed. Grabe), pp. 105, 106, 262.)
Two Tatians are almost always confounded as one person in Church history, although there was at least a century between them. The older Tatian was a Platonic philosopher, born in Syria, and converted to Christianity by Justin the Martyr. He was the author of a Discourse to the Greeks, became a Gnostic, and founded the sect of the Tatianists. The other Tatian was a native of Mesopotamia, lived in the fourth century, and wrote in very bad Greek a book called Diatessaron, supposed to be based on four Gospels, but what four is quite conjectural.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894