A village grew up inside this hillfort, including the church of St John the Baptist. In 1402 the Devil appeared in the church during a terrible thunderstorm, taking on the form of a grey friar and ‘behaving himself verie outrageouslie’ according to Holinshed’s Chronicles (written in the 16th century). The nave and a part of the chancel were destroyed. Cynics will put this down to the relative height of church, the fort being the highest point in Essex, and thus vulnerable to lightning and eldritch storms, rather than to any devilish qualities of the fort itself.
Mentioned in Westwood and Simpson’s ‘Lore of the Land’ (2005) p255.
It seems to be originally in Thomas Walsingham’s ‘Historia Anglicana’, written at about the time the event is supposed to have happened? You can see it here, and if you put the latin into google translate (I’m afraid my latin talents won’t run to it otherwise), you will hear about Unspeakable Terror of the parishioners and flashing globes of lightning.