A critical strike is a particularly strong weapon or spell hit that deals greater than normal damage. Critical strikes occur on very high natural dice rolls. Undead, oozes, and constructs cannot be affected by critical strikes and other forms of precision damage.
Weapon Critical Strike CalculationEdit
All weapons in DDO have a Critical Roll property listed in the item tooltip. In parenthesis is a summary of how the weapon critical works, and after that is the traditional PnP D&D critical notation. In the summary, the chance to threaten a critical strike preceeds the slash, and the crit weapon damage range follows it. In the traditional notation, the range of natural die rolls to produce a threat preceeds the slash, and the critical strike damage multiplier follows the slash. For example, a Long Sword reads "Critical Roll: (10%/2-16) 19-20 / 2". This means that a Long Sword produces a threat 10% of the time (on a 19 or 20 die roll), and when confirmed, the weapon deals double damage, making it's weapon damage 2-16 instead of the usual 1-8. Not that, like most information in a weapon tooltip, this does not take into account any changes from the player's abilities, feats, buffs or enhancements.
(Main article: Confirming a Critical Hit)
On the correct natural roll to produce a threat, a second attack roll with the same modifiers as the attack that produced the threat is rolled against the same target DC. If that attack roll would also hit the target, then the threat is said to be confirmed, and the original attack becomes a Critical Strike.
Once it has been determined that a weapon attack will critically hit, damage is rolled, and the weapon's base damage, the weapon's damage from enhancement bonuses, and the weapon's damage from the player's Strength modifier are multiplied by the weapon's critical strike damage multiplier. Certain other sources of damage may also be increased by a critical strike, refer to those sources' tooltips for information.
Advanced Critical Strike CalculationsEdit
Now that you know that a threat for which the original attack roll would miss is also a miss, and that many different abilities and creatures can prevent treats from being confirmed, we can explore which types of critical rolls are the "best". While we can confidently say that a greater critical strike damage multiplier is better than a lesser one, and a wider threat range is better than a narrower one, it's not always clear which of the available weapons has the best damage output given its base damage critical roll.
Greatsword versus Great AxeEdit
The Greatsword deals 2d6 slashing damage and has a critical roll of 19-20 / 2. The Great Axe deals d12 slashing damage and has a critical roll of 20 / 3. As first glance, these would appear to be more-or-less the same. However, the Greatsword has an average base damage of 7 and its damage is much smoother, while the Great Axe has an average base damage of 6.5 and has much more inconsistent damage. Similarly, the Greatsword threatens critical strikes more often, but each confirmed critical strike is much less impressive, while the Great Axe threatens comparatively fewer critical strikes, but those that are confirmed can be quite massive indeed. Since we know that threats that would otherwise miss cannot be confirmed, we know that when attacking a target which you can only hit on a natural 20, the Greatsword effectively "loses" half of its threat range. This problem is further exacerbated when comparing Keen weapons. The math for this case is fairly straightforward: a Greatsword will produce more damage on average than a Great Axe would, unless you're fighting something with an AC that is 20 or more points above your attack bonus, in which case the Great Axe will produce more damage, on average. For the purpose of overcoming damage reduction when you neither have enough damage from Strength and enhancement bonuses nor the correct damage type, the Great Axe will hit near to its maximum damage more often than the Greatsword would, and is thus a better choice for facing the rare monsters with very strong damage reduction.
Evaluating Keen or Impact Weapons and the Improved Critical FeatsEdit
Obviously, the Keen and Impact enchantments are useless for characters with the correct Improved Critical feat and vice-versa because the two effects do not stack.
Beyond that, both are a clear DPS and Burst Damage increase against most targets. However, if you commonly find yourself fighting enemies with a high AC, these effects are likely to be partially wasted on weapons that already have a fairly wide threat range, particularly the Rapier, Scimitar, and the Falchion. Thus, these effects are most effective on weapons with a good critical roll, but for those weapons with the wide threat range, these effects run the risk of being superfluous.
Spell Critical Strike Calculation
Damaging spells are capable of a critical strikes as well, however, as most damaging spells do not use an Attack roll, the spell critical strike calculation is a bit different. When a damaging spell is cast, the game checks d100 ≤ spell crit chance. If the check passes, then all damage dealt to all targets of the spell is multiplied by the character's spell critical strike damage multiplier. By default, all characters have zero spell crit chance and a 1.5 spell critical strike damage multiplier. Both numbers may be increased through various enhancements or weapon abilities.