Defensive casting mode allows casting without triggering an attack of opportunity. However a Concentration check (DC 15 + spell level) must be made or else the spell is lost.
NWN2 Gameplay[]
The concentration check is a variable DC from 15 (cantrips) to 24 (level 9 spell). A concentration skill of 23 or more allows this check to be basically bypassed in all circumstances, since a roll of 1 is not an automatic failure.
Failure means the spell fails totally. For a full explanation of casting a spell see Casting a Spell.
Bonuses to Defensive casting[]
- Skill Focus: Concentration adds 3 to the concentration skill.
- Combat Casting adds 4 to any checks in defensive casting mode.
In almost all circumstances, Skill Focus: Concentration is a better feat to choose compared to Combat Casting. The 3 bonus to concentration works for the base concentration check against damage, DC being the damage dealt to you, separately for each attack that damages you during spellcasting, as well as for some other general skill checks.
Defensive casting at various levels[]
The amount of concentration generally rises a lot faster than the DC of this skill. At higher levels, AoO are more dangerous too. Therefore, at the higher levels we use this a lot more often. This is the DC and maximum raw ranks in concentration using a wizards spell progression:
Levels Chance of passing defensive casting DC spell 0 1 2 3 4 5 6 7 8 9 class max con.
rankDC 15 16 17 18 19 20 21 22 23 24 1 4 50% 45% - - - - - - - - 2 5 55% 50% - - - - - - - - 3 6 60% 55% 50% - - - - - - - 4 7 65% 60% 55% - - - - - - - 5 8 70% 65% 60% 55% - - - - - - 6 9 75% 70% 65% 60% - - - - - - 7 10 80% 75% 70% 65% 60% - - - - - 8 11 85% 80% 75% 70% 65% - - - - - 9 12 90% 85% 80% 75% 70% 65% - - - - 10 13 95% 90% 85% 80% 75% 70% - - - - 11 14 100% 95% 90% 85% 80% 75% 70% - - - 12 15 100% 100% 95% 90% 85% 80% 75% - - - 13 16 100% 100% 100% 95% 90% 85% 80% 75% - - 14 17 100% 100% 100% 100% 95% 90% 85% 80% - - 15 18 100% 100% 100% 100% 100% 95% 90% 85% 80% - 16 19 100% 100% 100% 100% 100% 100% 95% 90% 85% - 17 20 100% 100% 100% 100% 100% 100% 100% 95% 90% 85% 18 21 100% 100% 100% 100% 100% 100% 100% 100% 95% 90% 19 22 100% 100% 100% 100% 100% 100% 100% 100% 100% 95% 20 23 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%
You can also lookup increased concentration amounts - for instance, if using Skill Focus: Concentration, so adding +3, check the lines starting at 7 and onwards (although this won't match what spells you get at which level). At a total skill of 23 in concentration of course it always passes, so the table doesn't need to go any further.
You can usually count on a +1 or +2 point bonus from constitution, and a mage may also take Skill Focus: Concentration for a +3 bonus. A +5 bonus is normal, meaning the chances increase by a wide margin - the chances improve by 25% at all levels with this. Add in a magical item - like +6 gloves of greater concentration, or constitution increasing item, can increase it so all levels get automatic success at all levels.
The minimum bonus to gain a 100% chance of casting is therefore +11 concentration over your normal amount (at level 1, +11 means a total of +15 - a roll of 1 cannot fail to defensively cast a level 1 spell), although at medium to high levels, this lowers by these amounts:
Level Minimum bonus 1 11 2 10 4 9 6 8 8 7 10 6 12 5 14 4 16 3 18 2 19 1 20 0
At level 16, with just Skill Focus: Concentration, and no additional constitution modifiers (ie; a modifier of 0) means defensive casting will always work, even with level 9 spells. With items, it is easy to see when spells will always succeed.
It is worth noting that regular enemy attacks can also disrupt spell casting and that initiative is somewhat relative so you can be almost certain that if multiple enemies are attacking, some of them will have that opportunity regardless of your mode. In all cases a high armor class is a better guarantee that your spells will be successfully cast than defensive casting and since only one combat mode can be active at a time, offensive casters are wise to choose combat expertise or its improved variant instead.
Damage being the DC to standard interruptions, you can never have enough concentration so it is wise to maximize the skill and prioritize gear that adds bonuses. Fortunately monsters tend to do less damage than well built players. Again armor class prevents all interruptions so protect yourself.
If you do not possess other combat modes then defensive casting with enough concentration to avoid spell failure is acceptable.