Double sword
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Despite its ridiculous concept, the rare double sword is a desirable weapon for any Long Blades user looking to wear a shield. It has roughly the same stats as an eveningstar, though Long Blades scale with dexterity rather than strength.
Unlike the demon blade, the double sword is two-handed for Kobolds and Spriggans.
Strategy
Compared to demon blades
On most species, demon blades are mostly equivalent to double swords. If both weapons are at minimum delay, a +0 demon blade is 1% stronger than a +0 double sword against a 0 AC opponent. As the demon blade attacks faster, it is penalized more by AC, though it benefits more from slaying and auxiliary attacks.
Overall, the differences are too small to make an impact. A +9 demon blade is enough to win you the game, as is a +9 double sword. Generally, the sword you get first should be the sword that you use. (The sword you get first can benefit first from scrolls of enchant weapon).
For smaller species, the demon blade will usually come out ahead, because it does not use the shield slot.