āglǣca, m.n: hostile fighter, fierce combatant, great opponent. (AH-GLACK-ah / ˈa-ˌglæː-ka)

There aren’t any good word-for-word translations. Often translators use ‘monster’ but there is no word in Old English that means simply ‘monster’, and āglǣca can refer to (human) heroes as well as monsters. Grendel is an āglǣca but so is Beowulf.