From The Heretic Knowledge Vault
The real-world harpe, according to Wikipedia, was a regular feature in the bloodier parts of Greco-Roman mythology, and is thought to have had a resemblance to the modern Kukri, which has a curved blade. So also here: when we've seen it most clearly in Sarine's hands (for example, here), the Durus Flamma blade extends in a curve, although it's not as pronounced as with the real-life kukri. This is one of the things that sets it apart from a Spatha, another type of Durus Flamma sword. See also the sketch in the Durus Flamma article, as well as the Poe notes at the end of the article, where the Harpe spelling appears (although it's clearly Hape in the sketch).