Before the battle (After deployment, before rolling for first turn), roll a D6 for each unit of Rats-at-Arms or Vermin Guard with one or more Characters with this upgrade, and consult the table below to see what rules the bearer (not any mount) and Rats-at-Arms or Vermin Guard models in the unit gain for the duration of the battle.
1-2: Poisoned Attacks and Stupidity.
3-4: Thunderous Charge.
5-6: Lightning Reflexes. The unit immediately suffers D6 Strength 4 hits with Armour Piercing (6).