Effect:

"Dusk" "Chance to apply Wither on hit or block."

For starters, can block like old swords, reducing damage taken by 50% but taking the damage blocked as durability damage to the weapon. Must be in offhand to activate this. When you strike a foe or block an attack with this weapon there is a 25% chance to apply Wither I for 4 seconds to the foe. A spray of black particles happens when this activates. If you also have Nuit's Ire equipped it applies Wither II instead.

As a "Diamond Khopesh" it has the following stats:

Damage 5, Speed 1.4, Recovery 0.83