I've recently been able to do some work on combat animations. For a while now I've been trying to understand how the "fancy" melee on melee combat animations are triggered. This took me a while to figure out because I thought they were controlled by the combatanimations.2da. It appears that all other attacks are handled there except for this particular scenario. I believe this was hardcoded to act this way in the original game, and that is why they are not present in the combatanimations.2da.
If the attacking creature is using a basic melee attack, and the target creature is using a melee weapon, and they are both directly attacking each other. Then the attacker and target uses a variation of c*a*, c*d*, and c*p* depending on if they were hit or parried.
c*a1, c*a2, c*a3, c*a4, and c*a5 are for attacking (Attack Animation)
c*d1, c*d2, c*d3, c*d4, and c*d5 are for damaged (Target Response Animation)
c*p1, c*p2, c*p3, c*p4, and c*p5 are for parrying (Target Response Animation)
When the attack is queued up a random number is rolled returning 1,2,3,4, or 5 and then proper attack animation is then chosen. Once the attack result is calculated the appropriate response animation is chosen for the target.
Example Scenario:
So if the the attacker is using a single one handed weapon, and the basic attack random animation number is 4, then the attacker will use the animation c2a4. If the attack hits, the target will use c2d4. If the attack misses, the target will use c2p4.
Note: If you are confused by the naming convention for combat animation be sure to check out this post: (Analysis of the Combat Animations) by @JCarter426.
Here is a video demonstration of it in action.