Critical hit and instant death.
Weapons have a critical threat range, which indicates how likely they are to do serious damage. If the threat range is 19-20 it means that if the d20 used to hit scored a 19 or 20 a critical hit has occurred. If the creature struck had only 1 Hit Dice (HD) point it’s reduced to 0 hit points immediately, which probably causes instant death. If it has more than one HD point rolling another d20 and if that roll too was a critical then drop the creatures hit points to 0 hp, and repeat if there are yet more hit dice until a critical isn’t rolled or all threshold . Note that if the roll needs to have hit the opponent to score a critical hit unless a 20 is rolled. 20’s always hit.
The basis for this is that weapons do two types of damage. Instantly fatal damage such as stabbing through the heart that requires precision to kill. Attrition damage, for example bone crushing and muscle damage, allows the wounded to fight on but eventually slays the individual. Attrition damage is generally more easy to achieve.
Example
Drogan, a dwarf armed with a battle axe (+1 to hit, 1d10 damage, crit threat 19+) is fighting a Dreadtouch Megapede which has 12 hit points from 3 Hit Dice. He rolls a 20 to hit, which is a critical. As the Megapede has 3 hit dice it needs 3 crits to instantly kill it. He rolls a 19 but then a 14. The 14, although it hits, isn’t enough to drop his foe to 0 hp immediately. However he rolls 3d10 for damage, 2 for crits and one for the finally successful hit.
Weapons - a rough guide
Daggers are moderately easy to use, do low damage but have a high crit range.
+1, 1d4, 17+
Swords are moderately easy to use, do medium damage and have a good hit range.
+1, 1d8, 18+
Axes are average to use, do high damage and have an average critical hit range
+0, 1d10, 19+
Clubs, Maces and hammers are easy to use, do better than average damage and have a low crit range.
+3, 1d8+1, 20
Spears are moderately easy to use, do medium damage, have an average crit range. They gain a +1 bonus to initiative and have a range of 1 in hand to hand.
+1, 1d8, 19+
Missile weapons generally are average to use, do average damage and have a good crit range.
+0, 1d8, 18+
Spells that target single opponents generally have a average crit range (19+) while area affect spells cause critical hits rarely (20) if they roll to hit.
From this you can see why those people who are poorly trained in combat or are fighting well armoured foes pick crushing weapons like maces, which hit more often at the cost of reduced chances of instant death. Feats, powers, spells etc,. might increase the crit range of attacks.
Creatures and Hit Dice
Most humanoid use 1d6 to determine their hit points, humanoids are more vulnerable to damage than many animals or magical beasts, which use 1d8. Some creatures are brittle but difficult to put down, like skeleton warriors, who have 2 or more hit dice but only 1d4 for hit points, while others are tough but vulnerable, like zombies who use 1d12 for hit points but only have a single hit dice.
It’s perfectly reasonable for GMs to average and round up for creatures when writing adventures rather than rolling all creatures hit point that feature in a dungeon.
