Off-Guard
An off-guard creature can't react to danger fast enough due to being unaware or unprepared.
Stage 1 (Off-guard): The off-guard creature suffers disadvantage on Dexterity saving throws and cannot take free actions or reactions outside their turn. Attackers also have advantage on attacks made against an off-guard creature. If a creature is only off-guard against certain targets, it means actions and movement taken by those targets cannot trigger the off-guard creature's free actions and reactions but they can otherwise act normally.
Stage 2 (Exposed): As stage 1 but all attacks against them gain the unavoidable property.
Stage 3 (Vulnerable): As stage 2 but all attacks against them automatically hit. Attack rolls are only made to see if they get a critical hit.
Stage 4 (Helpless): As stage 2 but all attacks against them automatically hit, automatically crit, automatically pierce armor (if they don't already negate it), and automatically deal maximum damage.
Note: Flat-footed, Denied Dexterity Modifier to AC, and the bonus to attacks due to being Invisible or Hidden are all replaced with this condition instead. So attacking a creature while you are hidden from them simply means they are off-guard 2 against your attack. If you're concealed from them they're only off-guard 2 and if you're undetected, they're off-guard 3.
Note that the attack bonus from Flanking still functions normally. However, a creature that is flanked by 4 or more enemies is off-guard against all attacks that successfully flank them. So 3 or less creatures flank normally but 4+ also get the benefit of an off-guard victim.
Coup De Grace: Delivering a coup de grace is a full-round action (3 action Complex Attack activity) that provokes attacks of opportunity. It functions as a normal weapon attack except it automatically hits and the target must make a Fortitude saving throw DC10 + half damage taken or be reduced to 0 hit points automatically. If already at 0 or less HP, the target dies instead. This is a stage 1 death effect unless the target deals precision damage or crits, in which case it's stage 2 death effect. Mutilating a body is a stage 3 death effect.
Special: Anything that would cause you to "lose your Dex bonus" or "catch you flat-footed" causes you to be off-guard. All creatures are off-guard by default at the start of any scene until their first turn starts. As all creatures have this condition, the Initiative and Perception penalty is ignored unless another creature somehow doesn't suffer it.