Niche D20To cast a spell a creature must concentrate. If something interrupts their concentration while casting, they must make a concentration check or lose the spell. Concentration checks are rolled similarly to other skill checks, using their ranks, ability score modifier, and any other miscellaneous bonuses to the skill.
Casting Defensively
When casting a spell a creature leaves themselves open as they attempt the casting provoking attacks of opportunity. A creature may avoid this by making a concentration check to keep their defenses raised while casting the spell. The DC of this is 15 + the BCB of the spell. Should they succeed the spell is cast and does not provoke an attack of opportunity. Failure allows the spell to be cast as normal but allows it to provoke an attack of opportunity, which in itself may be subject to a concentration check.
Injury
Taking damage while trying to cast a spell requires a concentration check with a DC equal to 10 + the damage taken + ½ the BCB of the spell. Failing this check causes the caster to lose the spell without effect, though resources spent on the spell are still consumed as normal. This interrupting event strikes during spellcasting if it comes between the time the creature started casting and the time to complete casting spell, such as spells that take more than a single turn to cast. Damage taken to temporary hit points still counts as damage for the purposes of determining the DC of the concentration check.
When maintaining an already cast spell through concentration, or when taking continuous damage (such as a bleed effect) the DC instead becomes 10 + ½ the damage taken + ½ the BCB of the spell that is being cast or concentrated on. If the continuous damage dealt was the last damage the effect could deal (such as the bleed duration ending that turn), then the damage effect has ended and does not distract the caster.
Grappled, Pinned, or Entangled
If grappled or pinned the concentration DC is the grappler’s last MSB check used to establish or maintain the grapple + the BCB of the spell being cast. If entangled the effect is simply 10 + the BCB of the spell being cast. 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. This is usually detailed by the current weather or terrain the creature is on.
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.