Title says it all. Who were the Magharians? It seems like they were some group of Pre-Christianity Jews or something like that that lived in caves?
Any of many groups that lived in or used caves could be given the name Magharians (from Arabic).
The tenth century (CE) Karaite Jacob Qirqisani described them as a pre-Christian group with distinctive views about angels, creation and biblical anthropomorphisms.
According to Muslim scholar al-Biruni (973-1048) they used a lunar calendar and observed Rosh ha-Shanah and Passover only on a Wednesday.
In modern times, there have been some suggestions that those using the Dead Sea Scrolls and the caves at Qumran may have been Magharians, though there are several other theories.