The answer is "litotes"
Litote is a figure of speech, more precisely a figure of thought. It is used to slow down an expression by denying the opposite. It allows us to affirm something through negation. In Litote, the idea that the author wishes to pass is indicated by the denial of the opposite, this causes the speaker to attenuate an idea that could be aggressive, makes certain delicate situations more pleasant.
In this case, the phrase "That [sword] was not useless to the warrior now." either attenuates the idea that the sword was useful to the warrior.
