From D&D Wiki
Ulak Blades are the native weapons of the Furyans, designed for close combat / two-weapon fighting. Ulak Blades are always made in pairs from black Obsidian and are always of masterwork quality. The price above is for a single Ulak Blade
Obsidian is extraordinarily sharp increasing the critical range by one increment (i.e. Dagger 18-20/x2 instead of 19-20/x2) Obsidian has 15 hit points per inch of thickness and hardness 10.
Wielding a pair of Ulak Blades gives a circumstance bonus of +4 to Climb checks (or +2 if only one blade is wielded.)
Furyans treat Ulak Blades as martial weapons.
Monks may use this weapon as a special monk weapon.