Instant Death is a rule used in the Fighting Fantasy gamebooks Creature of Havoc and Slaves of the Abyss, and in the Advanced Fighting Fantasy series. A similar, but more restricted rule is also found in Star Strider.
Instant Death in Creature of HavocEdit
Because of your prodigious bestial strength in this gamebook, you are especially dangerous in a Battle. If you roll a double when rolling for your Attack Strength, you have struck your opponent so viciously that you kill them instantly, and the fight is over. You ignore your opponent's roll entirely in this situation, so that if they would have won the Attack Round through having a higher Attack Strength than you, they do not injure you, nor can they cause your death.
Instant Death in Slaves of the AbyssEdit
In Slaves of the Abyss, you start your adventure with a superbly crafted sword made of Fangthane steel. The quality of this sword means that in a fight you will kill your opponent outright if you roll a double 6 when rolling for your Attack Strength. As the battle "ends straight away", it is clear that your opponent's roll is irrelevant. If you fight without your sword of Fangthane steel, you lose the ability to inflict such deadly wounds.