[PEACH] Casting Defensively

[LingVudka]

I thought I'd put this out there for feedback. I thought Casting Defensively needed a look at and came up with this.


Casting on the Defensive:

The Concentration DC for casting a spell on the Defensive is 10 + spell level +5 for every creature that threatens the caster e.g. if three creatures threaten a caster attempting a 3rd level spell then the Concentration check to cast the spell is DC28 = 10+3+5+5+5.

COMBAT CASTING [GENERAL]

Benefit:
You get a +4 bonus to Concentration checks made to cast a spell or use a spell-like ability while on the defensive or while you are grappling or pinned. Also the DC increase from each threatening creature while casting on the defensive is reduced to +3 instead of +5.

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Worthwhile? If so is it balanced? Any suggestions on whether concentrating while grappling or pinned need adjusting?

[LingVudka]

I thought about the following for Concentration checks to make them a little more difficult and allow for interesting events.

Vigorous Motion
DC = 10 + spell level.

Violent Motion
DC = 15 + (2 x spell level).

Extraordinarily Violent Motion
DC = 20 + (3 x spell level).

Entangled
DC = 15 + spell level.

Grappling or Pinned
DC = 15 + spell level - for verbal only spells
DC = 20 + spell level - if the spell is verbal and has a material component that you have on hand
DC = 25 + (3 x spell level) - if the spell has a somatic component and a material component that you have on hand
All spell-like abilities require only a DC 20 Concentration check.

Multiple Checks
If you are subject to more than one concentration check then you would add half of the lesser check, rounded down, to the greater check.

e.g. you want to cast a 4th level spell on a bumpy wagon ride (vigorous motion DC14) but are threatened by 2 creatures (DC24) then the overall concentration DC = 31 (24 + ½ of 14),
if you were attempting to cast a 9th level verbal and somatic spell with a material component on hand while being grappled (DC52) in an earthquake (extraordinarily violent motion DC47) then the overall concentration check would be DC75.

Opinions please. Is this too harsh? Does this scale well?