What literary technique does the poet use in these lines from beowulf? that {sword} was not useless to the warrior now.

The answer is Lilotes. Lilotes is a literary technique, a figure of speech that uses a understatement, which is the use of a negative word to express the contrary, stating the affirmative without using the affirmative. In the example of the lines from Beowulf, "That (sword) was not useless to the warior now." "was not useless"  is the same as "was useful".

king hrothgar despair over the inability to defeat grendel