Either forked or here is fine. It is a different approach, but there should be enough cross-over between the two to make discussion of both fruitful.
Cool.
For example, one of the things that immediately occurred to me reading your post was that such a system was in some ways similar to the Mongoose Runequest II "combat maneuver" system. MRQII doesn't have any direct concept of gaining or losing "CAs" by being ignored. However, because CAs are spent on defense, anyone being ignored is automatically much more effective. Another big difference is that getting hit hard in MRQII is more a feature of bad luck or running out of CAs. That is, it punishes being engaged more than it rewards being disengaged--which is often a characteristic of systems that require active defense.
It wasn't clear to me whether your system does require active defense or not. I guess that I'm looking to either avoid active defense or at least do it differently than many systems have done, both for ease of handling and to keep well away from the "punish" aspect of being engaged. Specifically, I don't want to add something that makes a single creature being engaged by several opponents any worse than it already is--such as effectively locking down actions to defense. Getting ganged up on is already bad enough--thus the attractiveness of focus fire.
Consider a solo fight, such as a dragon. If the dragon can "engage" with is breath weapon, then he can keep things on an even footing.
The defense isn't active, at least not in the sense of MRQII. Every attack is automatically defended against by one of the target's ability scores, which essentially sets the DC of the task. Momentum can be spent to increase the ability score for the purposes of resolving an attack. This is a free action, so as long as there is momentum to spend one can do it.
Also unlike MRQII this system tends to encourage engagement, and therefore cinematic action, because successful active engagement is how momentum is gained in the first place. The exception tends to be in situations with very asymmetric engagement, particularly ranged attacks and ambushes that cannot be answered in short order. In such cases the best solution for the defender is to disengage and seek more fortuitous circumstances for the next fight.
It is also possible to handle gang-up situations elegantly because there are two basic ways to spend momentum on defense, and each is a different sort of gambit. One can spend momentum when an attack is declared, and the benefit lasts until that attack is resolved. Or one can spend it during one's turn, and the benefit lasts until the start of one's next turn. By spending it beforehand one can better handle being surrounded (it effectively replaces the notion of "fighting defensively") both by making attacks more difficult and by making oneself a less appealing target. The downside is that the momentum are committed before one knows if they were "needed" and so reduces the momentum available in future rounds whether they helped or not. On the other hand, by spending momentum when an attack is declared it is only spent when needed, but if unexpected situations pop up (especially additional attacks) the character might not have the momentum to deal with it the way they'd like. Deferring the choice can also be done to draw attacks away from other characters, since a heavy hitter will generally want to force an opponent to spend momentum and make an opponent who has already spent some on defense gain no further benefit.
These considerations are held in tension with the normal benefits of attacking creatures in the first place. So consider the following situation:
1 troll, lightly injured, with moderate momentum.
1 wizard, very injured, who has spent a lot of momentum to increase defenses until next turn and has none left for other purposes.
1 fighter, somewhat injured, who has a lot of momentum.
It is the troll's turn.
The troll is weak to many of the wizard's attacks, particularly ranged and mind-based attacks, which favors killing it right now. On the other hand, the wizard has very high defenses this round, so the overall damage won't be very high, and unless the troll spends all his momentum on the attack there is a strong chance he will fail to finish it off. That said, even if an attack on the wizard fails to kill it the wizard can't unleash its most devastating stuff for at least a round or two.
The troll is strong against the fighter (and everyone else) in melee combat, all things considered equal, but the fighter has a lot of momentum to spend and could potentially introduce a game-changing hindrance on its next turn. If the troll attacks the fighter he can probably force the fighter to spend a good amount of momentum to defend against the attack and blunt any future attacks, but in a few rounds the wizard will probably be a sizable threat again.
The players were able to shape this situation for the troll by spending momentum in a way which made focus fire (i.e. attacking the wizard) less palatable. If both characters had high defenses from spending momentum beforehand then attacking the wizard is the clear choice (low damage in either case, so may as well go for the kill). If neither had spent it then the wizard is still the clear choice (the wizard will need to spend it or die, and might still die). If the wizard did not spend it but the fighter did then the wizard is the blindingly obvious choice (the wizard will need to spend momentum or die and might still anyway, and the fighter essentially wasted his momentum). In the scenario as presented attacking the wizard is probably still the best call for troll, but it is an interesting decision which has implications for the rest of the fight. To me that speaks to the heart of tactics: a dance where one invites the opponent to make just a few more missteps than oneself.
One other aspect I like about having these two ways of spending momentum defensively is that it can support different play-styles without baking too much into the character build. For example, a barbarian may very well tend toward offensive power, and use momentum for defense only when deemed absolutely necessary. Or someone who wants to play a thoroughly defensive style can do so without making their attacks uniformly worthless. (I think especially of the 3e defensive expert, often a duelist, who was largely untouchable but did peanuts for damage or perhaps couldn't even hit at all. The PC's contribution to the party often felt more like a courtesy from the DM than an achievement. Just pointless survivability.) Players can find a style that suits them and the character, but still have the freedom to change tactics to meet the situation.
In your solo dragon example in my homebrew system the usual balance is that dragon attacks are so powerful that they require almost any PC to spend momentum to counter them (or die very quickly) while giving plenty of momentum to the dragon. Consequently the dragon tends to control the tempo of a fight, or safely withdraw if desired. Even if the dragon has no allies present it will usually have enough momentum to boost its lowest defense for the entire round, reducing the gang-up effect, even as the PCs must scrounge for their momentum. And although the breath weapon is potent, even attacks that target just one or two PCs are usually enough because the dragon can often ignore a PC for a round or two without much risk of that PC reaching a really dangerous level of momentum. And if the PC spends all its momentum on an attack it may very well regret doing so. (By way of example: a dragon with high momentum that spends all of it on a breath weapon attack is likely to kill about half the party outright, even if they were previously unhurt.) Solo fights therefore tend to feel very different from a normal fight because, despite ganging up, the PCs have trouble using momentum the way they normally can. Even when the PCs are doing well it feels a bit desperate. I like that: a fight with a dragon should never really reach a "clean-up" phase.
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Ainamacar , really like that momentum idea. I'm probably going to steal it, but I'm not sure for what.
Glad to hear it! I think it works conceptually for almost anything where one can imagine the characters getting into a psychological flow state (i.e. "being in the zone") or other situations where "success begets success" and the raw consequences of success fail to capture something intangible about the result. That was my starting point, at least, long before I had mechanics I liked. So far I've tried it for combat, social scenarios, and complex skill checks. It shares a lot in common with morale systems, so one could probably adapt it for that purpose too.