Based on the 4th edition rules
A group in a position to ambush or surprise their opponents can make a Contested Roll of Stealth (Ambush)/AGI vs. the other side's Investigation (Notice)/PER. The rolls should be Cooperative or Cumulative, at the GM's option.
A surprised party suffer -10 to their Initiative Roll (affecting their Initiative for the entire combat).
Alternatively, in some circumstances, the GM may rule that the surprised party cannot act in the first Round, or may disallowed certain Stances (like Full Defense), or may rule that surprised combatants are easier to hit. These options are much deadlier.
Attacks are made as a Weapon Skill Roll against the opponent's Armor TN. Modifiers are frequently added, e.g. -10 for ranged attacks against an enemy in melee range of the attacker.
The Armor TN is REF×5 + 5 + any other bonuses.
If the attack roll equals or exceeds the Armor TN, the attack hits, and damage is rolled. Weapons have a Damage Rating (DR), like 2k2; the attacker's STR is added to the rolled dice. (Some ranged weapons have a DR for the ammunition, and add e.g. the bow's STR instead.)
An attack with a weapon in your off-hand is at a penalty depending on weapon size: -5 for small, -10 for medium, and -15 for large. While holding a weapon in your off-hand, you also suffer a -5 penalty to attacks with your primary hand.
A character wielding two weapons gets a bonus to Armor TN equal to their Insight Rank.
Damage is measured in Wounds. Wounds characters suffer fill up the 8 Wound Ranks, after which a character dies. The first Wound Rank has Earth×5 Wounds, and subsequent Ranks have Earth×2 each. (Thus, a character at full Health can endure Earth×19 wounds before death.)
Wound Ranks inflict a penalty, increasing the TN of all rolls.
Wounds are healed by rest: for every night of rest, a character recovers Wounds equal to 2×STA + Insight Rank. Treatment with skills or spells may increase healing.
Falling from a height does 1k1 damage per full 10 feet fallen.
Characters in deep water need an Athletics (Swimming)/STR roll at TN 15 (or more, depending on circumstances) each minute to stay afloat. Once the roll is failed, the character can hold their breath for a number of Rounds equal to STA, and continue trying to make rolls to resurface. If still underwater at the end of those Rounds, they start to drown, are helpless and unable to take Actions and take 2k2 Wounds per Round.
On the first Round of a combat, characters can pick a Stance in Stage 1, before rolling Initiative. On later rounds, characters can pick a Stance in Stage 2, before their Actions.
Stances affect what Actions a combatants may take.
On their Turn each Round, characters can take Actions, limited by their Stance. Each character can take either one Complex Action or two Simple Actions, and Free Actions.
Note that characters at the Crippled Wound Rank need to spend a "higher" Action to take an Action; a Simple Action must be spent to take a Free Action, and a Complex Action to take a Simple Action. Complex Actions cannot be taken. Likewise, characters at the Down Wound Rank can only take Free Actions (and that requires spending a Void Point).
Free Actions: Draw small weapon, drop weapon/item, Move Action (5×Water ft.), pull out spell scroll, speak (up to 5 words).
Simple Actions: Activate kata, dismount horse, draw medium/large weapon, guard someone, Move Action (10×Water ft.), pick up weapon/item, put away spell scroll, speak (more than 5 words), stand up from Prone.
Complex Actions: Cast spell, make attack (melee/ranged), mount horse, skill use (non-Weapon Skill), string bow for use.
Generally, a character can only take each Free Action once per Round, but can take any number of different Free Actions.
An action can be spent to move:
A character may only move up to 20×Water feet in one Round (unless they have a special ability that increases this limit).
Terrain can cause movement penalties. Moderate terrain (tall grass, foothills, sandy beaches, etc.) reduce your effective Water Ring by 1 (to a minimum of 1) for the purposes of movement. Difficult terrain (dense undergrowth, hip-deep water, mountainsides) reduces your effective Water Ring by 2 (to a minimum of 1), and the GM may assign penalties (usually -5 or -10) to physical Skill Rolls.
Maneuvers are special Actions. They require the character to call Raises, increasing the difficulty of the Action. (This means their availability is also restricted by your Void Ring.)
Attacks strike the torso by default. A Called Shot to a specific leg or arm requires 1 Raise; one to a hand or foot requires 2 Raises; one to the head requires 3 Raises; one to an eye, ear, finger, etc., requires 4 Raises. The GM must judge the effects.
A successful Disarm does only 2k1 damage (from the jarring impact), but the attacker and defender make Contested STR Rolls. If the attacker wins, the target drops the weapon. At the GM's discretion, a wood-hafted weapon struck by a steel blade (e.g. sword on spear) may be broken.
If the first attack succeeds with the Raises, the character immediately (after resolving damage) gets another attack. This maneuver may only be used once per Turn.
If the attack is successful, damage is increased by half the difference between the roll total and the Armor TN (including the Raises), up to a maximum bonus of 5× Insight Rank.
This maneuver involves no attack roll, and is not available in the Full Attack Stance. Designate one person within 5 feet; until your next Turn, while that person is within 5 feet of you, their Armor TN is increased by +10 and yours is decreased by -5.
Each Raise increases damage on a successful attack by 1k0. (Add up the Raises together before applying any effect that reduces the number of Raises needed.)
This maneuver requires 2 Raises against two-legged opponents and 4 Raises against four-legged opponents. If successful, the attack deals damage normally, and the attacker and defender make Contested STR Rolls; if the attacker wins, the defender is knocked Prone.
Round One: Assessment: On the higher Initiative, both duelists assume Center Stance (lasting through Round Three) and make an Iaijutsu (Assessment)/AWA roll against TN 10 + (opponent's Insight Rank ×5). If successful, a duelist learns one of the following pieces of information, plus one per Raise:
If one duelist's roll beats their opponent's by 10+, they gain +1k1 on their subsequent Focus roll.
At this point, either party may concede the duel, recognizing their opponent's superiority. This is honorable, but may carry consequences.
Round Two: Focus: On the higher Initiative, the duelists make Contested Iaijutsu (Focus)/Void rolls. If one wins by 5+, they may make the first strike, and get one Free Raise on it for every additional 5 by which they beat the opponent's roll. If neither wins by 5+, they strike simultaneously.
Round Three: Strike: On the lower Initiative, the duelist who won the right to the first strike makes an attack roll (Iaijutsu/REF vs. normal Armor TN) with any Free Raises. This is resolved normally. IF the other duelist lives, they may make their attack. (In a duel to first blood, it is dishonorable to strike second if you were wounded.) If neither won the right to first strike, they strike simultaneously. (In a duel to first blood, neither is victor or defeated if they strike simultaneously and both draw blood; this is called a "kharmic strike.")
If the duel was not to first blood, and both duelists are still alive and able to fight, the duel transitions into a regular combat in the following Rounds.
Shugenja sometimes engage in a magical duel. Use the Iaijutsu Dueling rules, but substitute Spellcraft for Iaijutsu, and roll damage using the Ring the shugenja has Affinity for.
Grapples occur inside a normal combat.
A grapple is initiated by a Complex Action to make an attack roll with Jiujutsu/AGI against the target's Armor TN (but without benefit of any armor worn). If the attack hits, both attacker and defender are now in a grapple. A grapple can involve more than two combatants.
The attacker starts out in control of the grapple. The defender may try to gain control each Turn. At the start of their Turn, grapplers make a Contested Jiujutsu/STR roll to gain control, which lasts until the next character's Turn. The character in control has the following options:
Some weapons (mostly chains and some polearms) can be used to grapple opponents; the rules are identical, but use the Weapon Skill and basing Hit damage on DR and STR. If a character using a weapon to grapple loses control, their opponent gets 2 Free Raises to Disarm them on their Turn.
Fear effects have a Rank (from 1 to 10). To resist, roll WIL against a TN of 5 + (5 × Fear Rank). Add Honor Rank to the total roll.
On a failure, the character suffers a penalty on all rolls equal to -Xk0, where X is the Fear Rank. This lasts until the end of the encounter, unless the source of the Fear is removed earlier.
A character who fails the roll by 15 or more is overwhelmed, and either cowers or flees in terror.