Using spells to force Spellcasting Concentration Checks

[zza ni]

this is kinda new to me, something i completely glossed over, but in the 'Concentration Checks' section of the magic rules it state that beside rolling Concentration Checks when taking damage from spells (which i knew of, i like to ready magic missile to prevent enemy caster's spell casting), if you are being affected by a spell you still need to roll the check even if it's not a damaging spell:

spoiler:

"Spell
If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you’re casting.

If the spell interferes with you or distracts you in some other way, the DC is the spell’s saving throw DC + the level of the spell you’re casting. For a spell with no saving throw, it’s the DC that the spell’s saving throw would have if a save were allowed (10 + spell level + caster’s ability score)."

this open up a whole new playstyle and options for interfering with enemy casters (and things to be careful of when you cast your spells).

so a few questions that i'd like to ask:

1: can you double-dip? say casting entangle on a caster, as there is a separate roll for casting while being entangled (dc 15+level of spell being cast) will a caster need to roll twice when trying to cast spells while being hit with an entangle spell? once for the condition and once for being effected by a hindering spell? (after all the dc are different, unless the entangle dc happen to be exactly 15).

2: what would be a good spell to use for this? i'm looking for a wide area\hard to ignore\easy to effect (maybe auto effect) the target?

my go to would be some fear spell that make the target frightened if failed and shaken if saved, but is the -2 even effecting the concentration roll? it's not a skill check or ability check or a save.

3: how much of an effect must a spell have to be considered 'interferes\distracts' ? i guess even casting light on casters with light blindness should work, but where do we draw the line? will a stinky spell work? difficult terrain? gust of wind? (it last a whole round so the caster is having a very strong wind blowing straight at him...unless he moves)

[Pizza Lord]

1. Multiple instances that cause concentration checks should require a separate check for each. For instance, if you're bleeding or taking damage over time, like from an acid arrow you would need a check.
If you're also casting defensively to avoid an AoO, you'd need a second check.
If you were also on the deck of a ship tossing about in a storm, you'd need another check.

In the case of entangle specifically entangling the caster, I am not sure I would double those, however. I think the spell isn't necessarily affecting the caster insomuch as the effect of the spell is doing something that affects the caster. I know that doesn't sound right, but it's how I'd view a summon monster spell that created an orc that grappled the caster, I would just use the grapple concentration check and not also say that a spell is interfering and require another check. Similarly if the caster was in the area of effect of a wall of fire where they were taking damage, I would just use the damage roll (continuous damage), and not also claim they were affected by a distracting spell. Different GMs might call that differently, though.

2. Spells that have negative effects, even on a save, are a good choice. The cause fear or similar spells would be a good call. Even on a save, they will still be considered affected in a manner I could easily call being distracted (becoming shaken). The –2 penalty to ability checks from shaken would apply to a concentration check, as it is an ability check. It uses the ability modifier of the caster's casting stat. Similar to how an initiative check is a Dex check.

3a. (NOTE THAT THIS ANSWER APPLIES TO SPELLS SPECIFICALLY READIED OR USED TO INTERFERE WITH A CASTER AS THEY CAST)
I suppose you could use any spell that had a noticeable effect, at least physically or mentally on a target or creature in the area (or one that was suitably distracting; loud, smelly, bright, flashing, or otherwise pervasive in some way). However, if the spell already required a save when it was cast to avoid or otherwise not be affected, then I would probably rule the caster suitably undistracted. For example, if you cast a stinking cloud, that's suitably stinky and obnoxious (enough to make someone nauseated and unable to act in most cases), but if the caster passed their save, I probably wouldn't require a concentration check for being in it as they're casting.

That means it will almost always be a GM's call on some spells. For instance, casting light or another spell that illuminated an area probably wouldn't be distracting enough (except maybe against Light Sensitive creatures, as you pointed out), but a pyrotechnics might be. Bane might work, as the sensation and sense of penalty might be considered distracting (if even only a little), if you readied to cast it right as another caster was casting (and they failed their save).

Even a cure light wounds spell might be considered distracting by some GMs. For instance, if you're trying to cast and an enemy came up and healed you, even though it's beneficial, a GM might rule that the feeling of your wounds closing and healing up (let alone the knowledge that a supposed enemy just touched you distractingly) might be considered distracting. Some might agree or disagree, but I would definitely rule that the spell had to actually do something, ie. if the enemy wasn't hurt and there were no hit points to heal, it wouldn't be effective, but that's another GM call.

A GM might rule that casting haste on a spellcaster as they're casting might be distracting. Even though it's almost universally beneficial, they might claim that the sudden, unexpected increase in speed might interfere with their careful casting (assuming the spell had components that would be affected logically, like somatic). Even though there is no mechanical or rules effect that says casting while hasted is any harder or affected normally. It's within a GM's domain to say that the sudden unexpected speed burst is distracting enough 'at that moment of casting' to require a concentration check.

3b. In the case of an already present spell, it would depend on the spell. Technically any debilitating spell is a potential candidate, but I'll admit I don't usually require concentration checks for being under the effects of bane or even most fear effects, but I probably should start doing so.