The lines suggest that Grendel provoked terror in the citizens, making them very afraid.
Grendel felt that the Danes threatened his territory and so he attacked them without any mercy, devouring people and causing immense terror. No one could attack him, leaving the Danes completely terrified, insecure, and sad.
The impact that Grendel caused made the Danes lose peace and live in fear, unable to carry out their activities normally. This lasted until the rise of Beowulf, who managed to defeat Grendel and encourage peace in the region.
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