Shattered Kingdoms

I think that the best choice would be having the break be hard-coded and determined on the individual spells. Only two that I can think of should fall into this category for gameplay reasons: Holy word, and final strike. Maybe some cabal room-affecting spells too. Every other spell is fine because they're all targeted and unless you use some whack cheats you can't pre-cast a targeted spell.

Using a metric like concentration to determine this will cause a lot of unnecessary collateral damage, not to mention someone could just as easily release all and then holy word or final strike to avoid it.

What if holy word was given a buff and made into holy shout, affecting rooms around it!?

This is how I pictured you when you read the changes to necros.

But seriously, we're forgetting the walk-in, follow, cast technique. Sorc walks in with 2 charms and a pet in formation to protect him while he follows you and begins casting petrification. You try to get away, only to have him follow you. All you've accomplished is you've moved your statue a few rooms away from where it would have been.

EDIT: or you had the reflexes and quick wit to quaff WoR, or just recalled, or whatever. Yes, I understand you can still get away, but those spells could be more limited if they were broken upon moving.

Auto follow, newbie.

It's an incomparably bad idea to follow someone as a sorc and start casting an aggressive spell. At best, there's nothing stopping the person from auto following and walking away or recalling. However, if he trusts his fortitude at all he can dirt kick you, flee (You're second row), wait for the petrification to start combat and then flee again and you're front row. Or, if he's a smart melee character he has a reaching weapon out behind a pet, meaning that he can put you in the front rank and start hitting you in the face before you even finish casting petrification.

In short, if you follow someone and start casting an offensive spell, you are setting yourself up to get hosed.

Yeah, Edoras learned this the hard way VS Paddah!

What about holy bomb? Where you just throw a flask of holy water at a person from the room away, giving the skill boulder, for giants, an immense boost!

It just lands on you and blows up EVERYTHING.

TANTH?!?!!?!