Before Christianity... before Judaism..

[EGross]

I stand corrected, there is a slightly earlier work, around the 12th century (Midrash Rabba Bamidbar 16:25) that has one sentence mentioning that character:

Another exposition: [Moses talking to god who wants to destory the Jews]: "Do not do it, that the nations of the world may not regard you as a cruel Being and say: 'The Generation of the Flood came and He destroyed them, the Generation of the separation came and He destroyed them, the Sodomites and the Egyptians came and He destroyed them, and these also, whom he called My son, My firstborn (Exodus 4:22), He is now destroying! Like that Lilith who, when she finds nothing else, turns upon her own children, so because HaShem was not able to bring this people into the land... He has slain them’ "

Apparently, she eats her young!