[D20/OSR] Counter attack

This optional rule can be used with games that use D20 to hit and higher is better.

When attack roll is low enough there's a change that an opponent can do an immediate counter attack. When counter attack happens roll normally to hit. The change of counter attack depends on character's level or monster's HD.

The number listed for counter attack change is the die result attacker rolls.

Counter attack cannot be counter attacked.

Example: Fighter's counter attack change is 1-3. If opponent's attack roll's result is 1-3 he doesn't only miss but the Fighter gets immediately an attack against him.

CHARACTERS

Counter attack is different for character classes. If the character class in your game is not listed use the closest one.

Cleric (Not only healers and supporters but also warriors of light)
Levels 1-7 = 1 in 20
Levels 8-14 = 2 in 20
Levels 15+ = 3 in 20

Fighter (Trained to fight and good at it)
Levels 1-3 = 1 in 20
Levels 4-8 = 2 in 20
Levels 9-14 = 3 in 20
Levels 15+ = 4 in 20

Magic-User (They rely on magic, scrolls and potions instead of melee)
Levels 1-15 = 1 in 20
Levels 16+ = 2 in 20

Thief/Rogue/Specialist (They have many skills and can use some extra tricks if they get into a fight)
Levels 1-10 = 1 in 20
Levels 11-15 = 2 in 20
Levels 16+ = 3 in 20

Dwarf (Natural warriors with hatred on orcish fiends and giants)
Levels 1-5 = 1 in 20
Levels 6-15 = 2 in 20
Levels 16+ = 3 in 20

(Dwarves have 1 better for counter attack against "greenskins" and giants).

Elf (Capable of both magic and fighting techniques)
Levels 1-5 = 1 in 20
Levels 6-10 = 2 in 20
Levels 11+ = 3 in 20

Halfling (Small and nimble with an extra trick in their sleeve)
Levels 1-7 = 1 in 20
Levels 8-15 = 2 in 20

MONSTERS

Monster's counter attack change is based on monster's HD.

HD 1-6 = 1 in 20
HD 7-11 = 2 in 20
HD 12-16 = 3 in 20
HD 17+ = 4 in 10

(Monsters basically are like fighters but as fighters are trained they have a slight advantage over monsters in general.