Ohh, so you are looking for aesthetic effects, not mechanics.
The problem here is you need to adjust both if you want the look you are going for. If the sword just passes through the enemy and you just pull off sweet combos all day, it’s already too late
Something I found really annoying about an example you give, Assassin’s Creed 3, was the fact that so many of the moves he does to some of the regular enemies would most definitely be fatal, but the enemy keeps fighting unphased if you don’t keep hitting him over and over, and it just ends up breaking immersion. In reality, people don’t take hits and lose HP, they get hit and either die, or become incapacitated. So if you want a more dark, real look, first off, get rid of combos. Second, when you or the enemy gets hit, and it isn’t fatal, there needs to be consequences, like some form of injury or incapacitation. This is the anatomy of a real fight if lethal weapons are involved.
Once you understand that, look into something like this
While you obviously aren’t looking for comedy and want to make gritty game #9,000,001, the visual pacing is still important. This should give you some good hints about the visual aspects of action sequences.
Now, if you want to do this cheaply, you need to swallow the difficult fact that something like this, especially if it’s 3D, just isn’t possible to do cheaply, unless you know a guy who owns a mocap studio who will let you use it for free.