Combat

Dual wielding

Dual wielding(having a one-handed weapon in your primary and offhand) grants a +10% increased attack speed bonus and grants +15% increased chance to block. The attack speed bonus is applied manipulatively and not additive. Skills will alternate between using the primary and off-hand weapons to calculate their damage output- unless the skill specifically specifies that it attacks with both weapons at once(Cleave and Dual Strike).

How hit and damage are calculated

These are the steps that determine whether an attack is a hit or a miss and how much damage is dealt if that attack lands:

• First the game calculates whether the enemy will avoid the hit. The enemy’s chance to evade comes into play and your own accuracy comes into play. 
• Second the game checks to see whether the damage will be avoided by rolling to see whether the damage is blocked.
• Third the game checks to see if the damage is mitigated by applying physical resistance and damage reduction(from armor).
• After all three of those checks have been made the game then calculates how much damage you take. Non-chaos damage is removed from energy shield until the ES is empty and then removed from life. Chaos damage bypasses your energy shield and directly reduces your life.

Stuns

In Path of Exile ANY TIME a player or mob takes damage they have a chance to be stunned. Stun duration is also shorter than stuns in other games, the default stun lasts 350ms. A stun interrupts what a character is doing and plays a brief stun animation. Stun duration can be modified with stats such as increased block and stun recovery, increased stun duration, and similar modifiers.

The chance to stun and be stunned is determinant on the amount of damage dealt in relation to the max health of the target. So the more damage you can deal in relation to the max health of a mob the more likely they will be stunned. Similarly the more damage mobs deal to you in relation to your max health the more likely you will be stunned. There are also specific skills that cause the stun effect, like shield charge.

Accuracy

Accuracy is checked against the target’s evasion to determine whether an attack lands of misses. Chance to hit can not drop below 5% or increase beyond 95%(unless you take the keystone resolute technique)

Evasion

Evading an attack prevents all harmful damage and effects of that attack(such as freezing, burning, etc.). Only attack skills can be evaded. Spell skills cannot be evaded.

Evasion also is used to calculate whether or not you take a critical strike. If an incoming critical attack succeeds in bypassing your evasion to land, a second independent roll is then initiated to determine whether you can evade the critical portion of the attack. If that roll succeeds you will only take damage as if the critical attack were a normal attack. And again critical strikes from spell skills can’t be avoided via evasion.

Path of Exile’s evasion system is not fully random. Here’s an explanation from Mark of Grinding Gear Games:

Each entity in the world contains an ‘evasion entropy’ value, between 1 and 100. The higher this value is, the more likely they are to be hit by the next attack. The initial value is random.

Every time something attacks you, they calculate their chance to hit as a percentage. That value is added to your evasion entropy. If the result exceeds 100, you’re hit, and 100 is subtracted from the value. If the value hasn’t reached 100, you’re not hit.

Before anyone starts clamouring that they’re not getting their actual chance to hit/evade, let’s examine this mechanic in a bit more detail. Take the simple example of 100% chance to hit. Since you always add 100 to the entropy, it’ll always exceed 100, and thus always hit, which is correct. The case of 0% chance to hit can similarly be trivially shown to be correct.

So let’s look at 50% chance to hit. Since the initial value is random from 1-100, there’s a 50% chance that the initial entropy value is higher than 50%, in which case adding the 50 from chance to hit will exceed 100 and thus hit, and a 50% chance the value is 50% or less, in which case adding 50 will not exceed 100, and thus not hit. So the first hit has a 50% chance to hit, as it should.

The second hit also has a 50% chance to hit, but will never hit if the first one does – provided you’re only getting hit by things with 50% chance to hit you, you’ll evade every second attack, and be hit by the others.

Let’s say the initial entropy was 42. The first hit increases this to 92, and misses. The second raises it to 142, hitting, and then subtracts 100 from the value, leaving it back at 42.

I’ll leave other percentages as an exercise for the reader, but they all work out – if an attack has 25% chance to hit you, every fourth attack will hit, and so on.

The system is designed to reduce spike damage. So players relying on evasion for damage mitigation won’t see a string of 10 misses and then 5 hits that kill them instantly because that’s what the random number generator decided to do.

Blocking

A successful block prevents all damage and other harmful effects from the attack. Only attack skills can be blocked unless you take the passive skill points which allow for spell block. Chance to block is capped at 75%.

When attacks are blocked the game calculates whether the damage of the attack would have caused a stun. If it would have caused a stun then a brief blocking animation is played interrupting your other actions. If it wouldn’t have caused a stun you just don’t take the damage and no animation is played.

Armor and damage mitigation

Armor effects physical damage taken. Elemental damage and damage over time effects are not effected by your damage reduction value.

The amount of damage reduction is capped at 90%. The formula for reduction is: damage reduction = armor / (armor + 12 * damage)

So armor is more effective in its ability to reduce percent of damage taken for mobs which deal less damage per hit than for mobs who deal more damage per hit.

Energy Shield

As long as your energy shield is greater than zero then you have a natural 50% stun avoidance.

When you take damage the damage is subtracted from energy shield until it reaches zero before it effects your hit points. Chaos damage completely ignores energy shield and directly effects your hit points. Energy shield naturally recharges if you don’t take damage for six seconds. Increased energy shield cooldown recovery modifiers from passive skills reduce this recharge delay. A 100% increase in increased energy shield recovery would decrease the recharge time from six seconds to three seconds.

Critical Strikes

Critical strikes are determined on a per action basis. That means if you use a skill like cleave and hit five monsters at once, you will roll once for that attack. If it lands on a critical all five monsters you hit will receive a critical strike damage.

Characters start with a base modifier of 150% damage to their critical strikes. The multiplier can be increased via skills and items. Your critical strike chance cannot drop below 5% or exceed 95%.

Elemental Damage

Fire – Critical strikes landed by fire attacks cause the burning effect. Burning causes damage over time. Humanoids will flee when they are under the effect of burning. The amount of damage done by the burning is 1/3rd of the total fire damage dealt by the initial attack every second. Multiple stacks of burning can be active at the same time, however the damage from them does not stack, so only the highest-damage burning effect that is on the mob will deal damage.

Cold – Hitting mobs with cold attacks cause the effect of chilled. Chilled enemies move, attack, and cast 30% slower. Critical hits with cold damage cause the effect of Frozen. Frozen enemies cannot perform any action except to drink a flask.

Lightning – Critical strikes with lightning damage cause the enemy to become shocked. The shocked effect can stack up to three times. Under the effect of shock players and enemies take 40% additional damage for a maximum of 120% additional damage with three stacks of shock.

Chaos – Chaos damage ignores energy shield and reduces life directly. It is not considered elemental damage(but I threw it in this section anyway, deal with it!)

Burning, chilled, frozen, and shocked are known as status ailments. The duration of these status ailments scale with the amount of damage dealt:

Shocked: 276ms per 1% of max life dealt as lightning damage

Chilled: 138ms per 1% of max life dealt as cold damage

Frozen: 100ms per 1% of max life dealt as cold damage

If the duration of a status ailment is less than 300ms then it is ignored completely- the effect is not applied to the target.

Charges

Certain skills grant charges. These are the little red, green, and blue bubbles you see floating around players. Each charge lasts a short duration before its effect ends, and gaining more charges resets the duration for all of the charges effecting the player.

Endurance charges(red) grant +5% physical damage reduction and +5% elemental resistances per charge.

Frenzy charges(green) grant +5% attack speed and +5% cast speed per charge.

Power charges(blue) grant +50% critical strike chance per charge.

The maximum number of charges can be increased through the passive skill tree and certain skills, like flicker strike, consume a charge when cast.