Game Mechanics

Melee Splash
Melee splash is an instant AoE, with no Next Hit Delay, that damages all enemies 360° around the target. The main target does not take splash damage. "Damage is rolled once (deadly strike included), but CB is rolled individually for each enemy hit in melee splash. Resistances still apply for each monster though." - Canight

These are included in the splash:
 * Skill damage
 * Converted damage (Concentrate)
 * Normal attack damage (the values you see when you select the regular attack as your skill)
 * Elemental and physical damage charms and gear
 * Critical Strike and Deadly Strike (still only applies to the physical damage)
 * Life and Mana leech (reduced by 1/2, and only applies to the physical portion of the attack)
 * Crushing Blow

These are not included in the splash:
 * Missile damage (such as the Nova portion of Power Strike)
 * Stun
 * Knockback
 * Open Wounds
 * Any other on-hit effects

Melee Splash Examples
Let's take a look at a couple of examples. First, let's take an Amazon:
 * Lightning Strike for 1-10000 lightning damage and 1-10000 chain lightning damage
 * Normal attack damage of 1000-5000 (900-4900 of that is physical)

The total splash damage would be 1001-15000 damage.

And if they crit then the splash damage would be 1451-17450.

The 1-10000 damage from the Chain Lightning is not applied.

What about a Barbarian:
 * Concentrate (300% ED, 50% conversion)
 * Normal attack damage of 1000-5000 (900-4900 of that is physical)

First the physical portion of the normal attack damage is boosted by Concentrate's ED% bonus: 900-4900 becomes ~1800-9800

Then 50% of the physical damage is converted to magic

The total splash damage would be 1900-9900 [100-100 elemental damage + 900-4900 physical + 900-4900 magic]

And if they crit then the splash damage would be 2800-14800

Splash Radius
The base player melee splash radius is 3.33 yards but this can be extended by certain skills and items. The splash radius will increase every 20%, so there are breakpoints at 20%, 40%, etc.

There are also a handful of other things that have melee splash. These are all static and cannot be increased.
 * "Splashing" map modifier: melee splash radius is 3.33 yards
 * Golems: melee splash radius is 2.66 yards
 * Skeletons: melee splash radius is 2 yards
 * Wolves: melee splash radius is 1.33 yards

Melee splash and ranged weapons
There are a few gloves (The Hand of Broc, Bloodfist, and Steelrend) that have melee splash on them and they can be used to give ranged weapons, like bows and crossbows, melee splash.

This allows shapeshifters to use a ranged weapon in place of a melee weapon, while still gaining the benefits of melee splash.

This does not mean that ranged attacks (i.e. shooting with a bow) will have splash damage. Melee splash is always limited to melee attacks.

Critical damage
Critical Strike and Deadly Strike have been rebalanced to improve itemization - they now each cap at 75% instead of 100% and multiply damage by 1.5 instead of 2.

There are 4 different types of 1.5x damage stats:
 * Deadly strike: found exclusively on items.
 * Critical Strike: used in the following skills - Critical Strike, Hunger
 * Crit Mastery (melee): used in the following skills - Javelin and Spear Mastery, Claw Mastery, General Mastery, Polearm and Spear Mastery
 * Crit Mastery (throw): used in the following skills - Javelin and Spear Mastery, Throwing Mastery

(An attack is only able to be melee or thrown, therefore only one Crit Mastery type will be taken into account.)

To calculate the chance that one of these stats rolls a crit:

1 - ((1 - MasteryCrit) * (1 - CS) * (1 - DS))

As an example lets take an Amazon with:
 * 30% Crit Mastery from Javelin and Spear Mastery
 * 52% CS from Critical Strike
 * 25% DS from gear

1 - ((1 - .30) * (1 - .52) * (1 - .25)) = ~75% chance to crit

The theoretical maximum combined crit chance is ~96%

Crushing Blow
Crushing Blow is a percentage chance to reduce the target's current life by a fraction. The fraction applied depends on the target and the type of attack. Crushing Blow also caps at 100% chance -- any extra will have no effect. This is all unchanged from vanilla, however there are a few things that have changed.

The melee life reduction is now the same as the ranged reduction:

There is also now a cap of 65% when fighting against Champions, Uniques, Super Uniques, and Bosses. So as soon as these monsters are at 65% life, then Crushing Blow will no longer do any damage.

Breaking immunities
What will break immunities:
 * Battle Cry ( Physical )
 * Arctic Blast ( Cold )
 * Poison Creeper ( Poison )
 * Amplify Damage ( Physical )
 * Decrepify ( Physical )
 * Lower Resist ( Fire / Cold / Lightning / Poison )
 * Conviction ( Fire / Cold / Lightning )
 * Sanctuary ( Magic )
 * Inferno ( Fire )
 * Static Field ( Lightning )

What will not break immunities:
 * -enemy resistance on gear
 * Cold Mastery
 * Fire Mastery

Enemy resists above 99% (immunes)
Just like in vanilla, the effectiveness of -enemy resistances is reduced when they are applied to enemies who are immune. In vanilla it was reduced to 1/5, but in PD2 this is instead reduced by 1/2. Even after the immunity is broken, immunity breaking skills still only have 1/2 effectiveness.

As an example lets take a enemy with 110 fire resistance, and lets apply a level 20 Lower Resist (-30%) and a level 10 Conviction (-30%).

The correct calculation looks like this:
 * -30 / 2 = -15 &emsp;[apply the 1/2 penalty to Lower Resist]
 * -30 / 2 = -15 &emsp;[apply the 1/2 penalty to Conviction]
 * 110 - 15 - 15 = 80 &emsp;[take the final -resist values and apply them to the enemy]

It does not work like this:
 * -30 / 2 = -15 &emsp;[Lower Resist at 1/2 effectiveness]
 * 110 - 15 = 95 &emsp;[immunity is broken with Lower Resist]
 * 95 - 30 = 65 &emsp;[apply -resist from Conviction]

It's worth stating again, even after the immunity is broken, immunity breaking skills still only work at 1/2 effectiveness. However this is not the case for -enemy resist on gear, Cold Mastery, and Fire Mastery. While these things are not able to break immunities on their own, they do work at 100% effectiveness after the immunity is broken.

Enemy resists between 0% and 99%
Works identically to vanilla.

Everything works at 100% effectiveness

Enemy resists below 0%
When taking the enemy below 0% resistance you also have to apply a 1/2 penalty. This also applies to gear and items with -enemy resistance. Although things work a bit differently compared to breaking immunities.

As an example lets take an enemy with 25 cold resistance, and lets apply a level 20 Lower Resist (-30%), a level 10 Conviction (-30%), and -Enemy Cold Resistance (-12%):
 * 25 - 30 = -5 &emsp;[Lower Resist]
 * -5 - 30 = -35 &emsp;[Conviction]
 * -35 - 12 = -47 &emsp;[-Enemy Resist]
 * -47 / 2 = -23 &emsp;[apply the 1/2 penalty to get the final enemy cold resist]

Or to put it another way: first the full value of the -resists is applied ( 25 - 30 - 30 - 12 = -47), and then if the end result is below 0 then the result is reduced by 1/2 (-47 / 2 = -23).

Resists can be reduced to a maximum of -100%

Note: The 1/2 reduction only applies to damage dealt by players and pets. Negative resistance from monsters (i.e. Conviction, curses, etc.) are always at 100% efficiency.

Item drop Basics
Lets look at a high-level example of how the drop process works.

Mephisto is killed! What does the game do to determine your reward?
 * 1) calculates the number of items to drop (max 6)
 * 2) selects item types
 * 3) selects item qualities
 * 4) selects certain unique or set items (when applicable)
 * 5) selects item properties (when applicable)

The number of players only affects item #1, the number of potential drops. It has no affect on the quality of the items that drop (this is where magic find comes into play). Now the question is, how does the number of players affect how many items drop?

Any time you do something that drops loot (opening a chest, killing a monster, etc.) the game selects from a list of treasure classes (TC) that will decide what kind of items will drop. Each TC has a list of items (or other TCs) that can drop, as well as a probability that that given item will drop. There is also a number of "picks" for each TC that decides how many potential items can drop. The last piece of the puzzle is "NoDrop", which is a probability that a pick will drop no items. NoDrop is the only parameter that player count affects.

There's a lot that goes into how the new NoDrop value is calculated when more players join the game, but the short version is: the higher the player count, the lower the NoDrop chance (potentially reaching 0 with enough players).

There are some TCs that start with NoDrop at 0. These are not affected by player count, since 0 is the lowest possible value.

What's different in PD2?
The baseline drop rate in PD2 is roughly equivalent to vanilla's p5/5 (5 partied players in a game) for solo play, and scales up per player until it's slightly above the equivalent of vanilla's p8/8 when there is a full game of 8 players. This means that overall NoDrop for all TCs is significantly lower, and it becomes easier to reach a NoDrop of 0 with fewer players.

You still get loot increases from more players like vanilla, but the difference between a solo player and a group of 8 is overall much smaller.

Rune TCs are also the same as in vanilla, so rune drop rates are the same. Miniscule differences may exist in some cases due to new valuable items such as Larzuk's Puzzlebox being able to drop.

How does PD2 compare to LoD?
It's hard to directly compare the two since there are hundreds of TCs, and PD2 has even more than LoD (mostly because of maps). But we can compare a TC from LoD to a TC from PD2 and see how the NoDrop chance has changed.

Lets look at the TC called Swarm 2 (H). In this example, the first number is the amount of players in the game and the second number is the amount of players in the party. So p3/2 means 3 players in the game but only 2 in your party. If you used the /players 8 command in single player that would be equivalent to p8/1.

Chance for NoDrop to be selected by the "pick" (i.e. the chance that no item drops):

LoD
 * p1/1, p2/1 = 83.33%
 * p2/2, p3/1, p3/2, p4/1 = 69.14%
 * p3/3, p4/2, p4/3, p5/1, p5/2, p6/1 = 57.63%
 * p4/4, p5/3, p5/4, p6/2, p6/3, p7/1, p7/2, p8/1 = 47.92%
 * p5/5, p6/4, p6/5, p7/3, p7/4, p8/2, p8/3 = 39.02%
 * p6/6, p7/5, p7/6, p8/4, p8/5 = 32.43%
 * p7/7, p8/6, p8/7 = 26.47%
 * p8/8 = 21.88%

PD2
 * p1/1, p2/1 = 40.85%
 * p2/2, p3/1, p3/2, p4/1 = 16.64%
 * p3/3, p4/2, p4/3, p5/1, p5/2, p6/1 = 6.68%
 * p4/4, p5/3, p5/4, p6/2, p6/3, p7/1, p7/2, p8/1 = 2.75%
 * p5/5, p6/4, p6/5, p7/3, p7/4, p8/2, p8/3 = 0.99%
 * p6/6, p7/5, p7/6, p8/4, p8/5 = 0.33%
 * p7/7, p8/6, p8/7 = 0.17%
 * p8/8 = 0%

Key drop rates
This is a common enough question that it probably makes sense just to show what the drop rates are.

There's very little difference between p1 and p2-p8 so group key farming isn't really necessary.