Remember how I said I'd think about things for a few days and then decide how I'll handle this in my own games?
Well, it turns out that a few days is more than three months. Ah well, it's not like anybody was waiting for me.
I'm about 60% satisfied with these. Feedback would be greatly appreciated.
Rules For Fighting While Hidden
The are four levels of hidden-ness. The two highest represent being properly hidden, and barring Aspect stuff you can't reach them in combat. You have to start the fight hidden to be that well-concealed.
Most of the time, you only need to care about Not Hidden and Blocked, which are simple to manage. Truly Hidden comes into play when people enter the fight concealed and their opponents fail their initial Alertness rolls to find them. Hidden comes into play when someone who's Truly Hidden does something to reveal themself.
0. Not Hidden
This is what you are by default. Not much to say here.
1. Blocked
You benefit from a block which protects you from attacks and from attempts to Assess your traits. It's just a regular block.
2. Hidden
Nobody can see you, but people know you're around somewhere. They also have at least vague idea of what zone you're in. You can be targeted by actions, but people need to guess which zone to target and you can use your veil strength/hiding roll as a defence roll. Your actions are secret.
3. Truly Hidden
Nobody even knows you're present. Nobody can do anything to you. Your actions are secret. You can ambush people.
Being Found
You may roll Alertness or Investigation to find hidden characters as an action. When you do, everyone whose veil strength/hiding roll you beat is now Not Hidden.
If you're Hidden or Truly Hidden and you make a ranged attack or do anything else that affects anyone else, you become Hidden automatically. If you're within three zones of your opponents, they get to know your zone. If you're not, they know which direction you're in. In addition, everyone else gets to make an Alertness roll to discover you without taking an action.
If you're Hidden or Truly Hidden and you make a melee attack against someone in your zone, you become Not Hidden automatically unless you Take Out your target. If you Take Out your target, treat it as a ranged attack.
If you're Hidden and someone tries to hit you, and they picked the right zone, you may defend normally or use your veil/hiding roll to defend. If you defend normally, you become Not Hidden. If you use your veil/hiding roll to defend against a single-target attack and get hit, you become Not Hidden. If you use your veil/hiding roll to defend against an area attack, your defence is Mediocre (+0). But you stay Hidden, and you don't have to tell anyone they succeeded in hitting you. If they picked the wrong zone with a single-target attack, they just roll against your veil/hiding roll and if successful learn that you're not there. If they picked the wrong zone with a zone attack, they roll and nothing happens.
If you're Hidden or Truly Hidden and you move, you normally become Not Hidden. But you can roll Stealth to move 1 zone (2 or 3 with Supernatural or Mythic Speed) without being obvious. When you do, everyone else can roll Alertness to notice your movement. If you win, you stay as hidden as you are and you don't have to tell people where you moved. If they win, you become Hidden and they know which zone you moved to. If they win by 3 or more, you become Not Hidden.
At the GM's discretion, if you have a magical veil up and would become Not Hidden, you may become Blocked instead.
Hidden To Some
It's possible to be hidden in such a way that some of your enemies know where you are and others don't. But anyone who sees you can point out your position without an action, so this will very rarely happen. If you get into a three-sided fight or something and it happens, just track your hidden-ness separately for each opponent.
Perception Skills
At the GM's discretion, perception skills may modify actions taken against concealed characters.
Modifiers
At the GM's discretion, the lighting and other environmental conditions may modify people's rolls.