The Ulster Cycle tales that describe Cu Chulainn, most importantly the Tain Bo Cuailnge, survive in medieval manuscripts, chiefly the Book of the Dun Cow from around 1100 and the Book of Leinster from around 1160, written many centuries after the Iron Age setting the stories claim for themselves. This gap is the center of an ongoing scholarly argument over how much of the material preserves genuine pre-Christian oral tradition, carried forward by professional storytellers over hundreds of years, and how much was shaped, added to, or invented outright by the Christian monastic scribes who actually wrote the surviving copies down. Both manuscripts show clear signs of scribal comment and revision, including notes where a copyist records disagreement between different versions of the same episode available to him, which is itself evidence that the tradition was not fixed even by the time of writing.
The warp spasm, Cu Chulainn's battle transformation into a monstrous, barely recognizable state, is described with real variation across the different recensions, some far more grotesque and detailed than others, consistent with a story reworked by successive tellers rather than copied unchanged from a single ancient original. Modern scholars generally treat the cycle as a medieval literary construction built from older narrative material rather than a transparent window onto pre-Christian Ulster.