The following are rules for areas of effect such as blasts, radiuses, and how these interact with combat maps.
Attacks which cover a multi-square area. Select the square you want to target first, and then measure out from the target square. For example for a 2 sq. radius you select the origin square and measure out 2 squares from the origin. In the case of a line or cone, use the user as the origin, and measure out from one corner of the square.
If a solid or semi-solid barrier would block you from drawing a line to the target square, the square is unaffected with the barrier blocking the area of effect. It is up to the GM if the barrier is still standing after the area attack (such as in the case of a wall being demolished from an explosion).
When making attacks which cover an area, you roll an attack roll once and test it against all opponents (and allies if they would be affected too) in the area and resolve the effects separately against their Defenses. If the area attack instead relies on those within the area making a Skill check or other similar check against a DC, each one makes their own separate skill checks against the DC.
Grenade-like weapons and other similar explosives cover a "burst". When doing a burst, choose square you want to affect, measuring out from there. Spells similarly list a “6 square radius burst” instead of "6 square radius" if they use the burst rules.
In the case of grenades and "splash" weapons, they deal half damage to those they hit outside of the original target square unless otherwise specified.
Line of sight affects the ability to utilize abilities, enabling characters to be able to target enemies with abilities or determining whether or not they can be seen when performing skills and tasks.
Poor lighting, gas, smoke, foliage, and many other environmental effects can grant one concealment, making it harder for them to hit you. This otherwise represents a non-physical barrier preventing the ability to see or see clearly, but you can tell their general location. Concealment imposes a -15 penalty on attacks.
You're able to Hide while in Concealment.
More severe than regular concealment, total concealment occurs when one has line of effect to an enemy, but not line of sight. This applies if a character is forced to guess where the opponent is (such as due to invisibility). This is a -50 penalty on attacks.
You're able to Hide while in Total Concealment.
These are barriers which impede line of sight but don't outright block it, granting defense bonuses to the one utilizing the physical barrier for their benefit.
Partial cover is when only part of your body is being concealed by a solid object, such as a pile of boxes blocking you from the waist down. This grants you a +2 bonus to Physical Defense.
Cover is when you have trees, walls, carts, and piles of boxes covering most of your body however it isn't blocked completely. This grants you a +5 bonus to Physical Defense.
You're able to Hide while in Cover.
If you do not have line of sight to the target (you can’t draw a line from your square to theirs without crossing a solid barrier) they have total cover and can’t be targeted until they leave total cover or you acquire line of sight on them. This also applies to the one with total cover, meaning they must leave the cover in order to begin attacking you again.
You're able to Hide while in Total Cover.
Some types of terrain impede movement, deal damage to those walking over it, or cause other special scenarios to occur.
Broken ground, steep stairs, or even climbing up mountainous terrain can provide difficulty when moving. It costs twice as much in movement to move through squares of difficult terrain. Creatures larger than 1 square pay the cost of difficult terrain if part of their token lands upon a square containing difficult terrain while they're moving. It doesn't cost additional squares to move into difficult terrain however, only moving through or moving out of difficult terrain.
At sea, boats become difficult terrain when traveling through particularly rough waters, making it more difficult for crew to operate.
Moving through the squares of allies doesn’t impede you, however the same can’t be said for moving through an enemies’ square. Trying to move through an enemies’ square requires you to succeed at a Tumble check, the check's DC is equal to the opponent's Physical Defense.
Though not necessarily involving movement but instead the state of being knocked down, prone is common state. Being prone imposes a -4 penalty to you attacking with melee weapons, and grants enemies attacking a prone enemy a +4 bonus to hit you with melee weapons. Enemies attempting to hit you while prone with a ranged attack takes a -4 penalty to hit instead.
If you are prone behind a short wall or similar barrier that would grant Partial Cover, it instead improves to Cover.
Damage is typically easy to understand as a concept; a creature takes damage and dies when it reaches a set amount of negative HP. The rules presented here are instead damage rules that are outside of the standard rules, and require additional details.
Damage which is nonlethal is made in an attempt not to kill someone. This includes hitting someone with the blunt of a weapon to knock them out, or using a spell to stun them. Nonlethal damage is counted on top of lethal damage, the only difference being that once a character reaches 0 HP due to nonlethal damage they fall unconscious and don't go beyond 0 HP. Instead they are unconscious for 1 hour per nonlethal damage over 0 they've taken. Characters who have been knocked unconscious through nonlethal damage can be woken up before the hours are up, counting themselves as being at 1 HP.
Initiative is rolled at the start of combat as a 3d10 + Reaction roll, and normally is simple in its execution, however some rules apply to initiative - especially in regards to a player manipulating their position in the turn order.
Sometimes a character receives horrible luck during the initiative phase, and rolls poorly. Yet, this doesn't mean they're stuck with this! Once per encounter at the end of the first round they act in, characters may choose to 'regain their footing' before a new round is started. Those who wish to reroll their initiative may do so, but if so they must take the new result even if it's worse.
On your turn you can choose to not act and instead delay your turn in order to take it later in initiative. You need to take your turn at some point during the current round, and if the round changes it's counted as you having not acted instead.