Combat Factors

Critical Hits

A critical hit is one that does significantly more severe damage to the target's HP. Usually these occur from a natural 20 on an attack roll. Some skills allow critical hit damage without a natural 20. 

NOTE: Your critical hit range is not the same as the "20 always hits". For example, a Fighter Champion subclass "Improved Critical" allows you to critically hit on 19-20, but if your attack modifier was +5 and the target's armor class was 20 and behind 3/4 cover (another +5), your roll of a 19 (+5) = 24 will still miss.

When calculating damage for a critical hit, take the dice used for rolling damage and double those dice. Added numbers are not doubled. (For example, 1d8+1d6+3 would become 2d8+2d6+3 on a critical hit.)  Additionally, any of these dice that roll a '1' are instead counted as if they rolled the highest number (so on a d8 a 1 becomes an 8).

Typically this will be all that is done with a critical hit, but in some cases the GM may choose to have you roll your attack again and if the attack again succeeds on that after-critical-hit roll, an injury may be incurred as determined by the main type of damage (see tables for Injuries).

Cover & Shields

Half cover provides +2 AC, three-quarters cover provides +5 AC. If an attack misses a creature only due to an AC bonus from cover, the attack hits said cover. If that cover was another creature, that creature may suffers the effects of the attack if it overcomes its AC.

• Medium shields are the default (+2 to AC, can bludgeon for 1d6). 

• Small shields (buckler, targe) add +1 to AC (+2 if DEX is 16+) and bludgeon for 1d4.  

• Big shields (kite, pavese) add +3 AC but give disadvantage on DEX checks & saving throws.

Lethal & Cleave Weapons

When attacking with advantage and using a weapon tagged as "Lethal" and both die rolls hit

• if the damage die rolls its maximum (e.g. 8 on 1d8) then add to that an additional die roll (+1d8) and repeat (no limit on repeats if max rolled).

When attacking with a weapon tagged with "Cleave" and the attack roll would hit a second target within 5 feet of the original target (hit its AC),

• the 2nd target receives base weapon damage as rolled (no ability modifier /bonuses /sneak /smite /mark /etc.)

Unseen

When you attack a creature that cannot see you, you have advantage on the roll. If you were unseen because of successful use of "Hide" then you give away your location at the end of that attack (so additional attacks will not have advantage from being unseen).

When you attack a target you cannot "see" you have disadvantage on the roll and if you cannot even determine its actual location, then you automatically miss.

(See also: Conditions, Vision & Lightning )

Surprise

Surprise rules apply when the target of an attack does not foresee (any) threat (and are therefore off guard).

This is usually because of an ambush scenario, where attackers often also benefit according to the "Unseen" rules above, but there may be other clever deceptions that can result in surprise, even without being unseen if the target does not perceive a threat. For example, being distracted or a sudden break-in. Surprise may be applied on an individual (not party) basis.

• Combat begins: Everyone (surprised or not) rolls initiative and takes turns in initiative order.

• Surprised creatures cannot Move or take Actions or Bonus Actions or make Reactions until the end of their first turn.

• At the end of a creature's first turn in combat, they cease to be "surprised".

• Surprise itself does not give advantage on attacks, but many scenarios will consider the attackers as "unseen".

Readying Armor

Note the time it takes to perform the following if caught unready (more than a few Actions means unlikely able to benefit):

• Don (put on) armor: Light (1 minute/10 rounds), Medium (5 minutes), Heavy (10 minutes)

• Doff (take off) armor: Light (1 minute), Medium (1 minute), Heavy (5 minutes)

• Ready or put away shield: 1 Action

(From Xanathar): Sleeping in light armor has no adverse effect on the wearer, but sleeping in medium or heavy armor makes it difficult to recover fully during a long rest. When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die) and MP pool. If you have any levels of exhaustion, the rest doesn’t reduce your exhaustion level.

Mounted Combat

Independent (not controlled), untrained or intelligent mounts have their own initiative and take all actions normally.  Controlled trained mounts act on your initiative and can choose an action from: Dash, Disengage, or Dodge. In all cases, movement provoking opportunity attacks allows targeting either you or the mount, assuming the attack can reach you. See also the Cavalry Master in the lists of Feats.

If your mount is moved against its will, make DC 10 DEX save or thrown prone 5 ft from the mount. If your mount is knocked prone, you can dismount with your reaction (and land 5 ft from the mount) – if you do not or cannot do this, you land prone within 5 ft of the mount. Other disruptions can require a STR (Athletics) check to remain mounted.

Lack of a saddle may impose disadvantage situations. A military saddle gives advantage to checks to stay mounted. Barding armor costs 4x gear and is 2x the weight. Lances can be used one-handed while mounted.

Two-Weapon Fighting

This form of combat allows you the benefit of having two weapons at the ready and the option of either from which to make your attack. You can engage in two-weapon fighting with weapons with which you are proficient, and

• using a light melee weapon in each hand (if using DEX/finesse attacks or Strength less than 13),

• a one-handed non-Heavy melee weapon in your main and a Light melee weapon in your off-hand (requires Strength 13+, must be STR-based attacks).

Additional Attack:  When you use your Action to attack with the weapon in your main hand, you can use your Bonus Action to make an additional attack using the weapon in the other hand. Damage from the weapon in your "opposite hand" will not add your ability (STR or DEX) bonus (unless negative) to damage. If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it.

(Some of the restrictions are reduced by Fighting Style: Two-Weapon Fighting and Feat: Dual-Weapon Expert).

Discussion: Two Weapon Fighting is debated on Reddit, etc. - I considered analyses such as this and this in these mechanics.

Grappling

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them. 

The target of your grapple must be no more than one size larger than you, and it within your reach. 

A grapple is not an attack, it is a contested check (you roll and target rolls):

grappler STR (Athletics) or DEX (Acrobatics) vs. target's STR (Athletics) or DEX (Acrobatics) (best)

• If the grappler's roll is higher, 

  • the target is grappled (by the free hand / device of the grappler) and

  • the target's Speed = 0 while grappled (note combined with Prone, cannot stand up)

• If the target is smaller than the grappler, the grappler has advantage. (If larger, disadvantage).

• If another attacks grappler+target, Disadvantage unless Light/Natural weapon (or don’t care who is hit).

• Escaping a Grapple. A grappled creature can use one Attack as an action to escape. To do so, it must succeed on:

escaper STR (Athletics) or DEX (Acrobatics) vs. grappler STR (Athletics) or DEX (Acrobatics) (best)

• • The grappler must expend its first Attack action (no check needed) to maintain the hold or target is let go.

  • Any time a creature escapes a grapple, it can use its reaction to opportunity attack the grappler.

• Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

Knocked Prone & Standing Up

After falling, or being knocked to the ground, you are "prone" (hands and knees) or "supine" (laying flat). These are the effects of being in that state:

• Moving: You must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.

• Combat: You have disadvantage on attack rolls, and creatures attacking you have advantage if within 5 feet of you (but more than 5 feet they will have disadvantage).

• Standing Up: Rising from a throw or fall is a difficult thing in the heat of battle. It takes training and skill to do so very quickly and even more so when faced with an aggressive adversary.

  • You must spend half your Speed to attempt to stand up from prone.

  • Standing provokes an opportunity attack from all threatening creatures in reach.

Underwater Combat

When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.

A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart). 

Creatures and objects fully immersed in water have resistance to fire damage.