Castle Nygramous
This was the stronghold of the sorceress Hellawes, probably located near the Chapel Perilous. Hellawes probaby is to be identified with Helaes de la Forest Perilleuse, who appears in Vulgate VII.
Although there may be many Forests Perilous (that seeming as much descriptive phrase as a proper name), for lack a better clue Castle Nigramous and the Chapel Perilous could be placed either in the Forest Perilous of Norgales (North Wales) or that of southeast Wales around Dame Lyonors’ Castle Dangerous.
I would prefer the more southern location, in order to keep Hellawes’ sphere of activity separated from Annowre’s. Conversely, there might be equal rationale for making Hellawes and Annowre neighbor sorceresses.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470