Fǽr-níþ

According to the Old English Dictionary:

es;

fǽr-níþ

m. A sudden or pernicious hostility, mischief; pernĭciōsa hostīlĭtas :-- Sorh is me to secganne hwæt Grendel hafaþ fǽrníða gefremed it is sorrow for me to say what sudden mischiefs Grendel has perpetrated, Beo. Th. 956; B. 476. fær-niþ