Flash powder can be thrown as an enemy charges the wielder (as an interrupt). The charger must take an immediate Initiative test in order to cover their eyes. If he fails, he is temporarily blinded and it counts as a failed charge. There is only enough flash powder for one use during the battle.
Lore
An ancient Dwarf creation, flash powder is used in mines to illuminate darkened fissures in the search for gold and other precious minerals. In Mordheim, small bags of this substance can be used to blind enemies, throwing them into disarray as you launch your attack.