The latest session of my 4th edition Dungeons & Dragons campaign ended somewhat chaotically, after the sorceress had cast a cloud of darkness between the group and the enemies, cutting all line of sight. In fact we had made an old school mistake: Not fully knowing the rules, we had made rules up, based on real-world logic. Decades of experience should have told us that this is fraught with danger, as real-world logic is hard to apply to magical situations like clouds of darkness or invisibility. In fact, one of the big advantages of 4E is that it saves groups from arbitrary decisions by running combat like a game with fixed rules. Sometimes those fixed rules contradict logic, for example you can be freezing and on fire at the same time; but at least you don't get players arguing for an advantage based on some pseudo-logic applied to a fantasy world.

Now I have read up, especially the Rules of the Hidden Club, and the blog posts of the Online Dungeon Master on Line of Sight and Effect and on the Hidden Condition. It turns out that the 4E sight rules are comparatively easy, if not perfectly logical: You always know where everybody on the battlefield is, except for a hard to get "hidden" condition you can only achieve through the stealth skill. You could say that this works by sound or other senses (Many years ago a player in a campaign was a druid who could transform into an otter. Discussing on whether she could follow an enemy by sense of smell, she said "I smell like an otter". Hilarity ensued.).

That solves a lot of the problems of the combat we are still in and will restart in the next session: The players were afraid that behind the cloud of darkness there would now be the group of enemies with prepared ranged attacks, triggered on the darkness going away. But as both groups know where each other are, that won't happen. Darkness only gives total concealment, so the two groups could even still exchange ranged attacks, just with a -5 penalty. So in future fights that cloud of darkness won't really separate the combatants and cause a break in the combat.

The rule also has profound effects on how to handle invisibility. One combatant of this fight is a drow rogue, with a power to make himself invisible. But invisible isn't all that much of an advantage if the rule is that everybody always knows where everybody is. However as a rogue that character has stealth, and I thus rolled a stealth check for him. Now he is hidden, and that is actually what people usually imagine when they think "invisible". No enemy knows where the hidden character is, unless he beats that stealth check with a passive or active perception check. And then he only has a rough idea of the location, unless he beats the stealth check by at least 10.

I guess the wizard of my group, who has the invisibility spell, won't like the new rule. But it saves fights from going into a limbo situation where the combatants don't know where their enemies are any more. And for the current fight I can just move the enemies behind solid obstacles, so they are out of line of effect as well as out of line of sight. So I don't have to rewind the clock or replay a turn.