I played through the Banner Saga finally, only a few years late. I did this with the intention of going through Banner Saga 2 as well, and wanting to finally play this game from my backlog so I could go right into the 2nd one. I have not yet touched the 2nd one. Whoops.
There's been a lot said about Banner Saga's narrative and art, and I'm generally in agreement that, like, yeah it has a pretty cool story and really good art and is generally great. But rather than have this become just a review where I say "Wow storytelling!" I want to deal with a mechanic that I think really makes the Banner Saga combat work: Willpower.
As a brief overview, Willpower is a resource that each unit has in a fight. It can be used pretty much as a general "Make whatever I'm doing stronger" resource, and also is required for most (all?) abilities. Willpower doesn't naturally replenish during a fight at all, but there is a mechanic to replenish some during a fight. Basically when you kill an enemy unit, you get a point of willpower (up to 5) that you can add to any of your units. Each unit has a maximum amount of willpower, which they start the fight with, and which is determined by their stats.
Ok, so basically willpower is this super-flexible "This needs to work" thing. If you need to move one more square to hit the enemy, or you need just one more point of damage to get through the armour to set up your other unit, or to even survive the next hit (Banner Saga also has a cool thing where health is damage, so you can block damage by attacking the health of the opponent), you can use Willpower to make it happen.
So, before I sort of look at the mechanic in more depth, I should probably talk about my relationship with willpower. At first, I ran into the Final Fantasy Potion Problem. You know the one where you end the game with 99 Potions, Hi-Potions, and Full Potions, because you know that you just might need them later on? Even when you're fighting a character who's name is "FINAL FORM SEPHIROTH" at the end of 80 hours of gameplay all revolving around Sephiroth? And how things probably would've been easier for you if you just, like, used potions every once in a while? Anyways, that's how I used Willpower. Really, I sort of used it as a combination of that and some sort of wrestling/anime finishing move thing where I used Willpower to really take out that final Dredge. Despite the fact that other than taking a turn to dispatch, if the last 3 enemies are at 1 strength, I'll probably be fine.
By the end of the game, I had swung too far in the other direction. I was burning through willpower like it was candy, and emptying the horn after every kill. This ended up working surprisingly well, but wasn't ideal.
So let's talk about this mechanic a bit.
Willpower is a flat +1 bonus to whatever it's enhancing. This is important, because of the way numbers for everything else work in TBS. TBS numbers generally range from 0 to 10, with some outliers (I had a hit against one enemy where I did 12 strength damage in a single hit). This means that a +1 can be 10% or less of a move's result, or can double it. What this ultimately means with this power curve is that Willpower is most powerful when dealing with the least powerful parts of a character. 1 Armour break may not be much, but 3 is a whole lot more. 2 health damage at the end of a round can be the difference between victory and defeat.
There's a hidden strength to this mechanic. Using willpower allows you to decide to essentially bypass or eliminate other mechanics. In normal combat, positioning is super important. But spend some willpower and maybe you can get around some positioning that wasn't great before. Armour breaking can be a long process of chipping away, unless you've got some willpower to toss at it. If you haven't broken enough armour, you might do tiny amounts of strength damage, but some willpower can make that damage more meaningful. Strength's cool health/damage thing means that if you're low on health you'll have limited ability to do anything meaningful in a fight, but hey, toss a willpower point in there and you can hit for substantially more.
Willpower as a mechanic basically says "If you're having a problem, this will bail you out". Because it will most often be used in these cases where you'd otherwise have a limited effect, e.g. some mechanic is getting in the way, it can be a tiny little thing that has a huge impact. This also makes using it correctly feel incredibly satisfying. All of a sudden you're snatching victory from the jaws of defeat, or turning a hit that would've been just another punch in a long slugging match into a killing blow.
I really like this mechanic. TBS has so many stats that do a single thing, and could really easily fall into the trap of just being a group of numbers smashing into another group of numbers. Having a general "use for whatever" resource that refills when you get a kill (thus encouraging its use to secure the kills) gives the combat so much more flexibility, and allows for more meaningful decisions to be made. It feels great to use, it lets players deal with some of the more unforgiving mechanics in the game, and it's just generally a really elegant thing.