The Diatessaron was a harmony of the four Gospels, that is, the story of the four Gospels rewritten as a continuous narrative, produced in the 2nd century, by Tatian, a Syrian Christian. There is disagreement about what language Tatian used for its original composition - Syriac or Greek. It was used in the Syrian Church, but was replaced in the 5th century by the four gospels individually, and is not extant except for small fragments.
There is also another Diatessaron, reportedly written by one Ammonius Saccas, to correct percieved deficencies in Tatian's one. (Note that this Ammonius Saccas is probably not the Ammonius Saccas who taught Origen and Plotinus, but rather a different philosopher with the same name.) None of this Diatessaron survives.
Empirical Models for Biblical Criticism. Ed. Jeffrey Tigay, Philadelphia: University of Pennsylvania Press, 1986