Scorpion's Sting

Each time this unit is selected to fight, if this unit contains a Striking Scoprions Exarch model, until that fight is resolved, replace the ability of this unit's Striking Scorpion Exarch model's mandiblasters with the following:
'Each time the bearer makes a melee attack that targets a non-VEHICLE unit, an unmodified wound roll of 5+ inflicts 1 mortal wound on the target in addition to any normal damage.'