So, you already know the stats that will be used in combat. Now you want to know how all that works. It's awfully simple, specially if you've played any tabletop games like Heroquest or the more recent Descent or Doom boardgames from Fantasy Flight Games. When we enter in combat mode, we somehow zoom in on the action that's taking place. Time is divided in rounds, and during every round every creature (PCs, NPCs, enemies) takes its turn. During a turn a creature can make two actions, either different or the same.
Moving is one action. A character may move a number of squares (if using a battlemap, otherwise a square equals an inch, which equals five feet in real life) up to its MOV stat. Humans (standard race for all three settings) have a MOV of 5. Jumping over a chasm requires spending two movement squares per square the chasm has.
Attacking is one action. You will use special combat dice with the following distribution:
If a character is adjacent to another he may make a Melee Attack; if not adjacent, but with a line of vision, it would have to be a Ranged Attack. The attacker rolls as many combat dice as the MAR or RAR stats, and counts the blood drops (which means successful attacks). The defender rolls as many combat dice as its MDR or RDR, and counts the number of crossed out blood drops (which means successful parries). Substracting successful parries from successful attacks gives the number of Hit Points the attacked creature suffers and that will have to be detracted from its HP pool.
When MAR or RAR is higher than one, you may divide the attacks against viable targets as you see fit.
The exclamation mark stands for a special symbol that may have different meanings. For example, in the Ajsalium setting, a magical weapon would hit with a symbol as well as with blood drops. Magical armour would save with a symbol as well as with a crossed out blood drop. In the Galaxtar setting a symbol may have a bad implication, signalling that a weapon has jammed.
Not all damage must be wound damage; there's stun damage too. Stun damage will be inflicted when fighting unarmed, or just by declaring the attacker wants to cause only stun damage. Unlike wound damage, stun one is added up; should the current stun damage ever be higher than the current Hit Points the creature falls unconscious.
And there are more possible actions. Opening a locked door or chest with a key is one action; as casting a spell in Ajsalium. Yet, difficult things such as picking up a locked door or casting a powerful spell take up to two actions, which means the character can't do anything else that round. On the other hand, some simple and quick tasks, such as opening an unlocked door, are move-equivalent, meaning that they just take the equivalent of moving one (or two) squares.
In Ajsalium there's magic. And magic is worked with the combat rules, too.
Casting magic is done by spending Mana Points: all spells will list the cost in MP to cast it, and also if it can be enhanced (increase the damage done, spread the reach, make it last longer...) by spending additional MPs. Magic users (wizards and druids for PCs, and some monsters) have a Mana Points score, representing their capacity to weave a spell and/or enhance it. They employ their MP as they see fit, but once the pool is depleted they won't be able to cast more spells until they replenish it. This can be done with rest (a good night rest replenishes all MP), or drinking mana potions.
Everybody will have a Magic Resistance, ranging from 0 (most common) to 6 (extremely rare). When affected by an unwanted spell, a creature rolls as many combat dice as its MR; if it rolls any symbol, it wards off the effects of the spell.
In Necronomicon, Hit Points are joined by Sanity Points. I still need to work a little on how to manage them, but here's what I've thought so far. Whenever confronted by some part of the Cthulhu mythos, a character must make a Sanity test (have yet to decide how to make them, but they will likely be like a leadership check in "Warhammer Fantasy Battles"). If he succeeds, the better for him. If he fails, he loses one SP; this alone makes him "shocked" (one point penalty in any roll) until he gets some time to rest and assume what's happened, and also makes him closer to madness, when SP get to zero (a mad character should be removed from play). The difficulty of the Sanity test, and the quantity of SP lost, will vary according to what part of the mythos is revealed; clearly seeing the great Cthulhu itself is far worse than spotting a deep one!