Dragonslayer Sword is just a Longsword with + a few d6 vs. Dragons, immunity to KD, aaand... I think that's all, apart from a +2 enhancement.Mercurial Longsword, as has been said, is one of the custom items that does not currently work with the weapon feats, such as Weapon Focus, Weapon of Choice, Weapon Finesse, and others (including the Duelist-only Precise Strike). Also in this list are the light pick, heavy pick, maul, katar, and a few other cool weapons.Any scimitar, however, should work. A falchion will not, but a scimitar will.