There are numerous combat situations not covered here, not to mention scores of creative ideas attempted by players to get themselves out of desperate situations. The following are but a few special situations that might occur.
Note that conditions that affect creatures often impose special situations. See Conditions for rules on cover, vision, invisibility, helpless opponents, incorporeal creatures, prone targets, and other conditions.
If you are exposed to a fire source, there's a chance you may catch on fire. Note that spells with an instantaneous duration don't normally set you on fire, since the heat comes and goes in a flash. If the GM or a spell says you catch on fire, on the beginning of every turn you make a DEX save (default DC 15) or take 1d6 points of fire damage. If successful, then you have managed to put the fire out. You may automatically extinguish flames by jumping into water or by taking a standard action to extinguish them.
Some spells and skills require concentration to succeed. If you are wounded while concentrating, you must make a Constitution save (DC = Damage Dealt) or lose your concentration and fail at that skill or spell in progress. Purely mental distraction provoke an Intelligence save to maintain concentration.
Any natural 20 on an attack roll is considered a critical hit. Base damage done by that attack is doubled (your main dice roll and modifier). Extra damage (fire damage, sneak attack, cold damage, etc.) is never doubled on a critical hit. A natural 1 is considered a critical fail. In some cases, the GM may devise a penalty for the character who fails spectacularly (a broken weapon, a fumbled and a lost weapon, accidentally shooting an ally, hitting himself, dropping a weapon).
When you fall, you take bludgeoning damage equal to half the fall distance (feet) in hit points. So, falling 40 feet inflicts 20 hp of damage. If you make a successful acrobatics check (DC 15), you take 5 hit points fewer damage, though you still end up prone if you take damage. If you are thrown against an object, you take a comparable amount of damage as well.
At times it might be necessary to attack with an item that wasn't designed to be used as a weapon. When using these improvised weapons, you do not get your combat proficiency bonus on attack rolls unless you have the Improvised Weapon Proficiency. When using improvised weapons, use the Category and Grade that best fits the improvised object. For instance, a tree branch might be considered a Medium Bashing Weapon, a broom might be a VL or L polearm, or a pair of scissors might be a VL knife.
You do not need to make Animal Handling checks for basic riding actions unless the GM feels the mount is so exotic as to require special checks. In combat, if you have the Mounted Combat Proficiency technique, then you can fight from your mount normally. If not, then you do not add your Combat Proficiency Bonus to attack rolls while mounted. Also, without the Proficiency, you need to make DC 5 Animal Handling checks to remain mounted when you or your mount take damage. You also need to make a DC 5 check to guide your mount with your knees, if you need both hands to attack.
Natural attacks are those made with a part of a creature's body. On a standard attack, pick one of these natural attacks to deal damage. On a full attack, you may use all natural attacks at their full attack bonus. Unlike weapons, the number of natural attacks does not increase due to a high combat proficiency bonus. Some abilities grant you an unarmed or natural attack as a bonus action. These attacks can be performed in conjunction with a standard or a full attack.
When making a full attack with a weapon, you may choose to use one or more of your extra attacks to make an attack with a natural weapon. In doing so, the extra attack still gains the normal -5 penalty, even if it's a natural weapon.
Unarmed strikes and kicks count as natural attacks and most humanoids can perform one bludgeoning attack with them at 1d4+str damage. Some humanoids have claws, which give them two attacks (on a full attack), usually with higher damage. If you have the Unarmed Combat Proficiency, you add your Combat Proficiency Bonus to your natural attacks, otherwise you don't. Monks are trained in natural attacks and have different rules for their use.
Generally, when a creature drops to 0 points, it falls unconscious and is dying. Optionally, the player dealing final bludgeoning damage may choose to knock the creature out instead. When doing so, the damage is still applied using all the normal rules, but the opponent automatically stabilizes.
When using a ranged weapon to attack into melee, you need to be careful not to hit your own allies. If you have a clear line of attack to your enemy, there are no penalties for shooting into melee. If a straight line between you and your target travels partially through a square that is occupied by another creature (including an ally), then your target is considered to have partial cover against your attack.
When firing through an ally to hit your target, the GM will determine how much cover your allies cause. Generally, though, if an ally is standing directly between you and an equal-sized target, the target will have half cover against your attacks. An ally that partially blocks a target will likely cause one-quarter cover. Note that the size of creatures makes a difference, so if a medium ally stands directly between you and a small or tiny target, the GM may rule that the target has three-quarters or even full cover. Likewise, allies in front of you might not block a huge creature at all.
When shooting into a melee, you make a standard cover roll against your target. If you miss the cover roll DC by more than 5 points, then there's a chance you hit the ally blocking your view. Roll an attack roll against that ally instead.
A splash weapon is a ranged weapon that breaks on impact, splashing or scattering its contents over its target and nearby creatures or objects. To attack with a splash weapon, make a ranged attack against the target's touch AC. Thrown splash weapons require no weapon proficiency. A hit deals direct hit damage to the target, and splash damage to all creatures within 5 feet of the target. If the target is Large or larger, you choose one of its squares and the splash damage affects creatures within 5 feet of that square. Splash weapons cannot deal precision-based damage (such as the damage from the rogue’s sneak attack class feature).
You can instead target a specific grid intersection. Treat this as a ranged attack against AC 5. However, if you target a grid intersection, creatures in all adjacent squares are dealt the splash damage, and the direct hit damage is not dealt to any creature. You can’t target a grid intersection occupied by a creature, such as a Large or larger creature; in this case, you’re aiming at the creature.
If you miss the target (whether aiming at a creature or a grid intersection), roll 1d8. This determines the misdirection of the throw, with 1 falling short (off-target in a straight line toward the thrower), and 2 through 8 rotating around the target creature or grid intersection in a clockwise direction. Then, count a number of squares in the indicated direction equal to the range increment of the throw. After you determine where the weapon landed, it deals splash damage to all creatures in that square and in all adjacent squares.
Some characters may fight with a weapon in each hand. When doing this, you do not apply your Combat Proficiency Bonus to your attack rolls unless you have the Two-Weapon Fighting Proficiency. If you are proficient, when making a full attack, you gain an extra attack with your off hand. As usual, when making a full attack, one attack is made at your full bonus and all others are made at a -5 penalty. Weapons in your off-hand must be light or very light. You cannot inflict precision damage (like sneak attack) with your off-hand weapon.
Underwater combat is made at disadvantage and half damage. With piercing weapons, you can attack normally with a successful swim check, a swim speed, or firm footing. A freedom of movement spell allows all melee attacks to function normally. Ranged attacks are not possible underwater.
If you have to hold your breath in water, you cannot cast spells with a verbal component unless you have the Silent Spell technique. Creatures that an breathe water can cast normally.
Fly spells do not grant you the ability to move freely underwater, but a levitate spell can cause you to move up or down in water the same as in air.