Demon weapons are characterized as requiring Demon Titanite for upgrade. Most of these are actually Boss Soul Weapons, but there are exceptions: some demon weapons are not boss soul weapons, and vice versa.
Demon weapons do not have any particular characteristics other than not allowing augmentation (the Dragon Bone Fist allows augmentation, but is not a demon weapon, since it requires Dragon Scales to reinforce it).
Demon weapons can be upgraded to +5 down the Demon Path. By doing this, it increases the weapon's base damage. This requires ×1, ×1, ×2, ×2 and ×4 Demon Titanite, for a total of 10.