Niche D20Associated Ability Modifier: Focus
A character usually must make concentration checks whenever you might potentially be distracted, such as by taking damage, while engaged in an action that requires your full attention. Primarily this includes actions such as casting a spell, or concentrating on an active spell. If the Concentration check succeeds, you may continue with the action as normal. If the check fails, the action automatically fails and is wasted.
As a skill concentration is primarily a focus of casters, but certain classes or actions may require a concentration check.
Motion
A creature attempting to make a concentration check within conditions of great motion takes a penalty to their concentration. Motion is broken down between Vigorous, Violent, or Extreme motion. In vigorous motion such as a mount, wagon ride, or a boat in rough water, taking a -2 penalty to concentration. Violent motion such as a boat in rapids or a storm takes a -5 penalty to concentration. Extreme motion such as an earthquake takes a -8 penalty to concentration.
Weather
Extreme weather can distract; conditions such as blinding rain sleet, hail, dust, and debris impart a -2 penalty to concentration checks due to their distracting nature.
Special
To cast a spell you must concentrate. If something interrupts your concentration while you’re casting, you must make a concentration check or lose the spell. When you make a concentration check you roll 1d20 + Concentration Skill (skill ranks, skill modifier, and any other miscellaneous bonuses to the concentration skill.)
Casting Defensively
When casting a spell you leave yourself open as you attempt the casting, you may avoid this by making a concentration check to both keep your defenses raised while casting the spell. The DC of this is 15 + the BCB of the spell. Should you succeed you cast the spell and do not provoke an attack of opportunity. Should you fail you cast the spell as normal but provoke an attack of opportunity, which in itself may be subject to a concentration check.
Injury
If you take damage while trying to cast a spell you must make a concentration check with a DC equal to 10 + the damage taken + ½ the BCB of the spell. If you fail this check you lose the spell without effect.This interrupting event strikes during spellcasting if it comes between the time you started and the time you complete a spell. Damage taken to temporary hit points still counts as damage for the purposes of determining the DC of the concentration check.
If you are maintaining an already cast spell or taking continuous damage (such as a bleed effect) the DC instead becomes 10 + ½ the damage taken + ½ the BCB of the spell you are casting. If the continuous damage dealt was the last damage the effect could deal (such as the bleed duration ending), then the damage is over and does not distract you.
Grappled, Pinned, or Entangled
If grappled or pinned the DC is The grappler’s last MSB check used to establish or maintain the grapple + the BCB of the spell you’re casting. If entangled the effect is simply 10 + the BCB of the spell you’re casting. These effects usually restrict the motions of the caster in some way making for a distraction to their casting.
Motion
A creature must make a DC 10 + ½ the BCB of the spell concentration check to cast within Vigorous, Violent, or Extreme; subject to the usual penalties to concentration for such motion.
Weather
Extreme weather can distract a spellcaster; conditions such as blinding rain sleet, hail, dust, and debris require a DC 5 + ½ the BCB of the spell concentration check to cast. This is subject to the normal penalties for such weather.
Action/Retry
Making a Concentration check usually doesn’t take an action or is part of another action. You may retry, though a success doesn’t cancel the effect of a previous failure, such as the loss of a spell you were casting or the disruption of a spell you were concentrating on.