The most important interaction is the attack interaction, which is used whenever a weapon, body part, or missile would come into contact with a target. The attacker first makes an attack roll (1d20) against the target's armor class (AC). If the attacker is proficient with the ability they are using, they add their proficiency bonus (P) to the roll. If this roll is equal to or greater than the target's AC (typically 10 + AV), the attack hits. If the roll is less than the target's AC, the attack misses. A natural 20 on the die is always a hit, and a natural 1 is always a miss. The consequence of a hit is described in the "On hit" section of the ability that's used.
If the hit is from a natural 20, then the attack is a critical hit and the effect of the ability "goes critical". If this changes anything about the effect, then it's described in the ability's "On critical hit" section. Any creature that is up may choose to be hit by an attack, even if the attack would normally miss. This is called "Taking the hit". They cannot choose to take a critical hit.
Shields have special rules regarding if a creature can attack. Shields always have the blocking property. But different types of shields have different rules that govern what can be used with them.
- Buckler Shields
- Any type of item can be held in an arm using this.
- Large Shields
- Any type of item can be held in an arm that's using large shielding equipment. But only small equipment can be used to attack or block.
- Tower Shields
- Only small equipment can be held in an arm that's using tower shielding equipment. But but no equipment can be used to attack or block.
Multiple Attacks
The first attack you make in a given turn has no penalty to hit. Each subsequent attack you make accrues some cumulative attack penalty (AP) which will apply to future attacks made in the same turn. The value of the penalty that's added is the AP of the equipment or body part you attack with. Unless otherwise noted, an attack has an AP of -3. This is the AP for missile attacks as well as any other attack where an -3 isn't stated. Your attack penalty resets to 0 at the end of each turn (not just your own).