Dungeons and Dragons Specific
Basic Combat (SRD):
Initiative determines the order of combat. After it’s done, it doesn’t change. The roll is 1d20 + DEX modifier. The highest goes first. SRD Initiative
To Hit vs Armor Class:
A “To Hit” roll is 1d20 + modifier. Armor Class starts at 10 for humans. Notice this means it starts as a 50/50 chance. It also means +1 is 10% more chance to hit.
A long sword does 1d8 damage. So, 5 HP is not much. 100 HP is a lot.
Rules of Thumb:
Normal person- AC 10, +0 To Hit, Initiative +0, 4 HP. (Lvl 1 Commoner)
Burgler- AC 14, +1 To Hit, Initiative +2, 10 HP, +1d6 Sneak Attack. (Lvl 2 Rogue)
City Guard- AC 15, +5 to Hit, +0 Initiative, 15 HP. (Lvl 2 Fighter)
Local Famous Warrior- AC 16, +10 To Hit, 30 HP. (Lvl 5 Fighter)
Critical Hits or streaks of luck can make a big difference in a combat. At low levels, this knocks out characters. At higher levels, it means characters are mush. There’s nothing much to do about it but recognize it exists and plan for the worst with story (maybe a last-minute heal or rescue or reinforcements or a newly-spotted escape route).
Example 1: 2 Commoners with spears do 1d8 damage each. Against a Lvl 5 Fighter, you would have to roll a ‘16’ to hit. You COULD roll that several times in a row by luck. If each roll is a ‘7’ or ‘8’, the level 5 fighter will only last about 2 combat rounds. Meanwhile, the level 5 Fighter should automatically hit every round for 1d8 damage. Rolling a ‘1’ for damage, neither commoner dies in the first round. In the second, the Fighter knocks out one Commoner. The next round, maybe the Fighter goes down. Based on BAD LUCK, 2 farmers kill an armored and skilled opponent.
Example 2: Spells can be the same. A Fireball or Lightning Bolt does 1d6 damage per level. The spread at level 9 is: 9-36 HP. It could be Highly effective, ineffective, or somewhere in between. A Rogue with evasion could make their Reflex Save on a low damage fireball and come away basically unscathed. OR, could fail their save against a high damage spell and be killed. Disintegrate is a great example at higher levels.