This is an example of litotes. This refers to a figure of speech that emphasizes a point by stating a negative to further affirm the positive. Litotes also tend to incorporate double negatives for effect. The way in which we can tell this is an ironic statement depends both on context and intonation. In this case, the author tells us that the sword was "not useless" in order to convey that it was actually extremely important.
Listening to music triggers the areas of the brain that have to do with spatial reasoning. Music can even alter the brain if it is studied from a young age. Research shows that musical training in children can improve the activity of important neural systems.
