Interrupting spells cast by monsters

One of the issues I found running my last campaign is that it is very hard for monsters to successfully cast spells. A beastman shaman or witch doctor may have enough cover from the other beastmen to manage it, but a dragon or sorcerous sphere will often be alone and "win initiative agaisnt 10+ characters or lose your turn" doesn't seem like a good deal for such a powerful creature.

At the same time, I like the riskiness of spellcasting - that spells are powerful, but a spellcaster can be locked down by a determined party.

With this in mind, I'm considering adding the following rule for my next campaign:

For a spellcaster to lose their spell to damage they must either fail a saving throw or take enough damage in one hit equal to or greater than their base HD (ignoring plusses and minuses).

Note that PC almost exclusive come from 1-1, 1 or 1+1 HD races, so for PC casters this replicates the base game rules, but if you want to stop a venrable dragon from casting, be prepared to hit it with at least 20 damage in one hit to stop it.

What do you think?

Interesting idea, I haven't encountered this problem yet. I'm not sure how you figure "base HD" though. A venerable dragon is just a "higher level" dragon, its base HD is 2HD (for a spawn). If you consider the level, then a 10 level mage would have to receive 10 hp for its spell to be disrupted. That could work too.

In any case, that seems to me to be a good angle to solve this problem.

I guess the distinction I am trying to draw is between Monster HD and class levels. High HD monsters tend to be larger and more imposing an doften fight alone, while PCs and NPCs have party dynamics to fall back on.

If all the characters are piling onto one monster, I'd use group initiative  

I'm not keen on changing initiative rules based on what kind of encoutner the party faces, and I'm not a fan of group initative in general, it creates too much of an alpha strike problem.

James, I think the problem is real. Your solution is quite interesting. I imagine some players might complain that while it makes sense that a dragon might ignore an arrow as being naught but a pinprick, it seems harder to justify that e.g. human-sized high HD monster can also do the same. This of course is because ACKS combines fighting skill and size/mass into one stat. To address this, you might tie it into monster size rather than monster HD. But your solution seems totally workable.

In my own campaigns, I tend to have my NPCs focus on buff spells. So e.g. the dragon casts mirror image, haste, and invisibility on itself.

How many man-sized but high HD spellcasting monsters are there? From memory, a lot of them are undead, and since they have unnatural vitality I can easily explain that. Beastman spellcasters can be dealt with by basing the damage threshold on a normal beastman, rather than the champion / subchief stats than casters usually have.

I see the problem in theory, but I'm not sure it exists in practice.

Personally I'd go with nothing hard and fast on this (HD is likely an ok base), casters aren't usually super frequent so manually setting a damage threshold could be the way to go.
Also keep in mind while some creatures may mechanically cast spells they would be more akin to innate powers than the typical humanoid casting rituals, so they are likely not even subject to interruptions (though not sure if the RAW does that or just my group).

I've had this issue as well.  I've tried two ways to handle it:

  • Roll d20 with a target roll of (damage taken) +
  • Allow a save  vs paralysis with a minus to the roll based on damage (1=-+,2=-2,4=-3,8=-4,etc)

I only allow this for those with unflappable casting. I haven't decided if I like either one. I definitely don't want to remove the 'shoot the caster' idea, and spellcasters need the limitation for fun and balance.