Several possibilities come to mind.

1.  Spell could have a maintenance cost.  They don't stay charmed unless you pay upkeep.

2.  There could be an ongoing chance of the creature breaking free of the charm based on its level.  For example, the % chance could be equal to the monster's level - a level 8 troll would have an 8% chance of breaking free each turn.  Thus, for very powerful monsters, the caster has a strong incentive to use the monster immediately or disband it.  This would prevent accumulation of large forces of high level charmed monsters.  For weaker monsters, who cares if the caster has a ton of level 1 spiders?  Probably not unbalancing. 

agreed. bc i already said it  <--- speaking of pokemon! OMG!    

the only problem i see with that is that a level 8 dragon (if we can even charm dragons...) would be so much more powerful than a level 8 troll. but they have the same percentage of breaking off the spell? I think mana upkeep- based on strength- would suffice. because that would make you balance how many turns you want to keep the unit without the frustration of losing the unit. That would just be irritating.

Which raises another question in my mind. Is this all going to be done on the main map or the tactical map? because that's very important. I would be far more irritated to have a unit break off a charm spell on the main map than I would if it broke the spell in a tactical battle. The latter might actually be kinda fun...    

It looks like we are all saying basically the same thing. Upkeep for holding monsters. Resistances based on magic-proofness, willpower and special considertions(aka no-brain). Couple talking about breaking free chances.

Are there going to be other sub-charm spells like charm undead? that could come in handy, but perhaps it should be a spell mainly for necromancers? TBH, i don't want to get into a DnD style where there are 15 different kind of charm spells, each one for a seperate situation, but you might want to split it into undead/organic/magical(golems and such).

I am of the belief that there are absolutely certain creatures that should be immune to charm spells, and I don't believe that charm spells should have an infinite hold, regardless of upkeep or no upkeep.  A charmed creature should get an increasingly better chance to break the hold of the spell over time if they did not resist it in the first place.  But that's just me being a party pooper. 

Thanks guys, a lot of great suggestions! We're working overtime to get some additional screens in for the beta, so I don't think we'll be touching charm again before Thursday, but defiantly keep the suggestions coming.

Oh, and right now I'm talking about the main-map version of the spell (non-tactical).

Could you say ideas that you think you'll keep (or seems interesting) ?

I don't like this idea.  Spells like what you have mentioned can imbalance the [vanilla] gameplay very easily.

Besides, it makes no sense at all. Charm = magical spell. Magical resistance protects a creature against magical spells. So basically this spell would ignore the creature's magical resistance?

- I agree that willpower and other stats should be taken into account. If you know your enemy can use this against you, counter spells and items are a must (though this currently IS just for roaming creatures).

- I can see per-turn mana costs adding data management overhead "why do I suddenly only have a max mana val of 5 when it was just 20?" But I can also appreciate forcing the player to make a significant, long term choice when casting it.

- I can see a 'timing-out' of a charm to be frustrating to the player, but understand the perceived necessity.  If the charm works, I don't like the idea of worry about it randomly un-working.  You'd have to be given fair warning, and perhaps the option of re-charming before the spell breaks.

- I like the idea of spell variations that alter stats...a 'Twist Creature', 'Soul-Crush', or 'Mind Domination' that givens bonuses and penalties to various stats of the charmed creature

All great feedback...gives a nice pool of ideas when we come back to polishing that feature.

If Stardock managed to reflect the Presence and Domination differences of Vampire: The Masquerade's disciplines, that would be awesome. Domination would be easy (take control, can end with the target losing its own initiative to act, needing direct input from the master) while Presence (Charm...) would be tricky as you don't really "control" the target (but it retains free will and can act by himself always).

I'd see 'Presence' as the simple ability for you to see through that unit's FOW.  That'd be pretty handy when keeping tabs on those wily Sovereign units

Nice trick.

Btw, if you do a maintince cost for a resource like mana - dont let it be deducted from the amount you have, instead reduce the total by that much.

Titan Quest did this for lots of abilities it made it much more fun to use. It was displayed as a solid 'used up' part of the bar - same mechanic but FAR more user friendly.

Its not exactly the same ofcourse but my point is what the player knows can make mechanics fun/unfun so good presentation is not 'icing' on the cake, its far more important.

Thanks for the reply

About willpower : it should be a stat used when you need to resist the spell and when you check for spell-end. Even if you control a strong willpower creature, it will have a greater chance of freeing itself.

Cost per turn : it shouldn't be different from the usual upkeep (gold or food or whatever ... if a "normal" spider cost 2 foods then a "charmed" spider should cost the same. Then if you want to control that archdemon who has an upkeep of 5 earth mana, 10 fire mana, 30 gold and 10 population per turn, then you have to prepare for such ;))

Timing-out : Why not warning the player when a charmed unit has over 75% (or even better :  a user-choosed stat) chance of breaking their binds ? Or something else : when a unit free itself, the unit will stay for one (or more .. depend on the willpower of tha charmer) round doing nothing. Then you have one round to try to charmed that unit again.

Altering stats : in fact control spells should be in 3 categories.

Total control : unit will never get its mind back, you just burnt it. But the controled unit has no brain, thus has less efficiency with brain-skills.

Domination : you control the unit, it's like yours, but has still its whole mind. You has total control, but domination will lessen with time AND with orders that are against its "way of life". Ask a spider to destroy a spider lair and it will have a great chance to fight the order. Order it to catch someone with their web and they will accept easily.

Influence : the influenced unit has lesser chance to strike your units, a chance to rebel each turn against it's owner and becoming a neutral unit (noone controls it), you'll spend less to bribe it, it becomes a spy for you, or talk to anyone how good you are (increasing diplomacy skill).

At least, some diplomacy tech should give you bonuses for influence. Military (or spell) should give you bonus for domination spell. Total control would need a fair level in life/death magic and civilization (how to keep alive something that has rotten from the inside)

Many have already voiced similar opinions but I'll just add mine as well to give it more weight.  The proposed system for charm is too simple.  You already have a magic resistance stat and this is the perfect place to use it.  It should be one of the major influences on the charm working.  Creature level or strength shouldn't.  For balance reasons I suppose it has to but I'd prefer seeing other ways of handling it.  Borrowing from another poster, a giant might be very strong but have a weak or gullible mind and be somewhat easy to charm for instance.  A zombie might be weak but would be completely immune.  I think you should also consider having special abilities enter in to this (I think you are going to have them, correct?).  For instance the Giant above could have the "Weak-Willed" ability giving a 50% bonus to charm spells (or any mental attack spell) against it.  The zombie would have the "Undead" or "No Mind" ability and be immune.  A system like this keeps things fairly simple and easy to understand but also allows for depth and flexibility.

What if there was no per-turn cost for roaming creatures, but if someone were to charm a player's units, they would have to have a per-turn cost, perhaps based on the opposing sovereign's power and/or targeted unit's power?

I also think the upkeep for charmed roaming units should be small or non existent.

This could be again one balance factor for small nations to help against larger nations.

Just think of a small magic kingdom which has charmed a large army and can now fight of the troops of much larger kingdoms.

Still this is a quite tough balancing topic. Maybe the upkeep should depend on the distance to the charmer???

This would offer smaller kingdoms a good opportunity, but is probably not easily traceable for the player, why he had one round an upkeep of 10 and the next of 20 by just moving a unit.

per-turn mana costs: Beside adding data management overhead, I do not see what kind of fun per-turn mana costs added to the game, especially if we are talking about a short term spell, charm, which only happen in tactical combat.

The better way is probably to pay the mana cost upfront, when you cast the Charm/Dominate spell.

When you cast "Charm them 2 turn" to 10 level 8 Trolls, it costs you 2*10*8 mana.  The UI tells you this cost when you mouse over the trolls.  If you agree to pay the mana cost, they are charmed for 2 turns.  The trolls will then gain a status of "Charmed 2 turns"

When you cast the same "Charm them 2 turns" to 10 level 2 spiders, you need to pay way less mana.   And of course, gamer can research Charm 3 turns, Charm 4 turns etc.  And there can be spells/equipment that reduces the cost of charming certain type of units.

any % for a charm to work is ok, the only issue is that on single player I would just save before I cast and then restart until it would work.  Then I would have a power house and stomp what I could.  I like the idea of a brake effect if you are low lvl and the creature is high lvl.  This way if I saved and restarted and eventually charmed the creature, it would only last like 1-2 turns, then I would have to go through the same hassle again when it broke.   That would make me think twice about charming it in the first place.  Especially if it wore off in a tactical combat, it would be a pain to save and restart 99 times lasting only 2 turns were the combat would take 5-6 turns.   

You can also restart the game until you have the perfect start position, make a mod that gives you an overwhelming advantage, or just plain use cheatcodes. All of these things can make the game a lot easier, and none of them are an 'issue'.

It is neither possible nor desirable to prevent such things.

Just to put it aside - obviously some monsters / armies have to have charm immunity / resist (resist can simply be added to monster level in calculation).

Several things:

1) To make level difference more significant you can use (Target lvl - Caster lvl) ^ X. you can tweak X or make it XML variable.

2) There has to be some upkeep for charmed monsters (mana / food / both)

3) a cool thing would be to charm your own troops - if you are low on resources but high on mana, use charm to reduce costs.

I would like to add to my above post by stating that I would rather there be no per-turn mana cost in general, but I thought I'd bring up the idea of having it for enemy players' units as opposed to not being able to charm enemy players' units at all

I see your point Netriak, but I must agree with AM_Shark. It does make gameplay more tactical if there is a limited numer of turns you can control the charmed creature. Especially if you have a 100% chance of charming it. Then you still have to wonder if you should charm it now or wait until a battle is close at hand, but then you might have to use other spells that would prove more useful. You can't go around charming everything and building defenses with everlasting monsters. That said, the other way you can prevent this "everlasting" side is by imposing a cost for charming creatures. I believe that ONE of those two options should be used, but putting both into place would be a little too much. In the end, either inforcing a cost (mana) per turn or a limited time (with possibly an upkeep penalty) would be a good solution in my opinion. I must say I prefer the second option though (with the upkeep penalty).

If you read my previous post, I was the one that proposed the idea that the duration of a charm depended on charm strength and creature/caster level. My point was solely against save-load abuse being something that Stardock should balance about.

Concerning infinite armies:

That has nothing to do with the charm spell. It is simply one way to acquire units. The difference is that instead of gold, you pay mana as initial cost. Instead of being limited by population, it is limited by availability of wandering monsters. But it is fundamentally just another way of acquiring units. I agree there should be an upkeep cost, but all units should have one. The more fantastical creatures may very well have a mana (As in the resource, generated by shards. Not unit based.) cost as upkeep instead of gold.

Having a low upkeep for neutral critters to be charmed might not be such a great idea. In any situation where a small kingdom could charm a bunch of them to hold off a larger kingdom, the larger kingdom could charm them quicker/earlier and have an even larger army.

It would also be cool if we had some brainsucker parasites. They could attack a unit in combat, suck their brains right out, and crawl into the skull themselves. No upkeep for the unit, just for the parasite. The unit gets large combat bounses since it dosen't care about living anymore and ignores daamge. The catch though is that the unit falls apart in some set amount of time. After all, the parasite isn't taking care of the body, just using it as a combat drone. And, we should be able to infect our own forces. Line up a group of peasents, pop a parasite into each. They go from being level 1 commoners to berserker golems with the parasite's vast experince guiding it. Would be hard on the local population but handy if you are in a pinch and need some troops quick.

Or we could have veteran parasites who could infiltrate generals/princes/leaders of other armys and lead them into ruin.

I dislike 'all or nothing' binary states, as they encourage save/reload play, and are generally either really broken, or really useless

Ditch the % chance of success (save/reload abuse, or randomly lucky power gain), and instead simply tie the charm power to its potential targets

A level 1 charms level 1 foes, a level 5 charms level 5 foes, etc

Play around with that, and if it works fine, leave it, if not, add an interesting downside to make casting the spell an interesting decision, instead of either a roll of the dice or an overpowered ability

eg, once charm breaks, the monster rampages against you with increased strength, or you suffer increased aggression from its relatives (mommy dragon don't like you), or you trigger random events as a consequence (troll rebellions due to your abuse of their kind!), or your foes empire gains temporary morale bonuses against you (he made me fight my brother!) etcetc