This is the 8368th most frequent Japanese word.
忌まわしい
Detestable; abhorrent; unpleasant.
In this sentence, '忌まわしい' is used to describe an event that is disturbing or shocking, evoking a sense of aversion.
その事件は忌まわしく、誰も話したがらない。
That incident is abhorrent, and nobody wants to talk about it.
Here, '忌まわしい' characterizes the witch as something people find loathsome or evil within the context of the story.
古い伝説では忌まわしい魔女が村を支配していたと言われる。
In an old legend, an abhorrent witch was said to have ruled the village.
In this instance, '忌まわしい' conveys a personal feeling of dread or negativity associated with a memory.