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I have a situation where several of my party members will be fighting in a treacherous canyon, with rope bridges, zip lines, and a lot of angry gnolls. Some of the gnoll clerics may be prone to casting obscuring mist if I am feeling cruel when we run the session.

If those gnoll clerics cast obscuring mist, I imagine it would hinder the PCs' attempts to see the ground and prevent themselves from stepping off the cliff in the midst of battle.

How can I handle their chance to fall off the cliff? I would presume moving slowly and carefully will help their chances, and I figure I should offer a reflex save if they DO make a bad step, but what sorts of chances should I use and what rolls will they have to overcome?

Changing the idea slightly, how would this be different if instead of casting obscuring mist, the gnolls simply killed all the lights? How would the PCs without darkvision handle the cliff then?

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If they are moving slowly and carefully, they should not be at any risk of falling accidentally. Obscuring Mist impairs vision beyond five feet, but a cautious character should be fine within five feet of an edge. In my personal experience, PCs so often act with caution that I would definitely reward them if they do.

The balance skill relates to movement in precarious situations. From the SRD:

A successful check lets you move at half your speed along the surface for 1 round. A failure by 4 or less means you can’t move for 1 round. A failure by 5 or more means you fall.

You can try to walk across a precarious surface more quickly than normal. If you accept a -5 penalty, you can move your full speed as a move action.

This provides us with a mechanical definition of 'moving carefully', and all but the clumsiest characters should be fine if they take 10.

If they insist on running, charging, etc. I would require a spot check to see the danger coming and then a reflex save to stop in time to avoid it. It's usually too late once you've stepped out into thin air to do anything about it.

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  • \$\begingroup\$ I like this answer - half speed eliminating the risk of one falling on his own misstep strikes me as kind, but still fair. I am uncertain of the relationship between this situation and balance however - if I understand you, it sounds like it would be a DC of 0 but they should still make the check? (Obviously armor penalties apply and they can't take 10 in a battle but still) \$\endgroup\$ – Trauma Advocate Feb 12 '15 at 22:52
  • \$\begingroup\$ @TraumaAdvocate The DC of a balance check depends on how narrow the available walkable area is and how uneven it is. In the case you've outlined, the DC is likely to be about 10 - which is to say, an automatic success for most characters if taking 10 is a possibility. \$\endgroup\$ – GMJoe Feb 12 '15 at 23:57
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A mist-covered cliff edge is similar to a covered pit trap. The d20srd page on traps says for pits that:

A character who fails to detect a covered pit is still entitled to a DC 20 Reflex save to avoid falling into it. However, if she was running or moving recklessly at the time, she gets no saving throw and falls automatically.

You probably want to be a bit more generous than this, because a mist-covered cliff edge is at least a little easier to spot than a covered pit.

Here's a hard question: even if the characters can't see the landscape, presumably the players got a clear view of it before the spell went up. How does one adjudicate that? One option might be to just remove all the figures from the Obscuring Mist zone, and ask for Survival or Listen checks if they try to navigate to a specific point in the zone. A badly failed check means they found the cliff edge. A badly failed check while running or charging... well.

You should give some thought to what happens if a character does fall off a cliff. Making a stupid error like walking the wrong direction should not get someone's character killed or even permanently removed from the battle. In the worst case, maybe they fall 40 feet or so, take some falling damage, land on a ledge, and have to climb back up.

Obscuring Mist is an interesting tactic because the mist is centered on the caster. If the gnolls were wizards or druids, I'd suggest they use Fog Cloud instead because that can be dropped on enemies at range.

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  • \$\begingroup\$ That's an interesting thought on the covered pit trap... it may not apply in this case because as you said they will probably get a look at the terrain before the spell went up (I am not feeling THAT cruel this week ;) ). Your point is well taken on creating ledges below however, I will be sure to toss some of those in there. \$\endgroup\$ – Trauma Advocate Feb 12 '15 at 22:56
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Don't worry about obscuring mist...

The 1st-level Sor/Wis spell obscuring mist [conj] (PH 258) says that its

vapor obscures all sight, including darkvision, beyond 5 feet. A creature 5 feet away has concealment (attacks have a 20% miss chance). Creatures farther away have total concealment (50% miss chance, and the attacker cannot use sight to locate the target).

First, that means a creature can see into its adjacent squares... not well enough to shank a conscious and mobile foe in the kidney, but well enough to distinguish ally from enemy, and, presumably, adjacent solid ground from adjacent empty space. Second, keep in mind that creatures don't, RoboRally style, commit to all their movement before making it; a creature needn't plot a course then take that course, heedless of his new knowledge of battlefield conditions. Instead, the decision either to continue to any other legal square or to stop moving comes after both assessing the effects of having entered the new square and determining what new information about the world entry into the new square has provided.

Hence a creature enveloped in the vapors created by an obscuring mist spell should no more accidentally fall off a cliff than he should accidentally throw himself into a wall. While that would be hilarious in Teenagers from Outer Space and a possible pedestrian fatality in Car Wars, that's not really a thing here.

That's not to say this isn't tactically dangerous, and, really, the spell obscuring mist if cast on plateau—unless one side has a way to see through the vapors that the other side lacks—should bring the fight to a standstill until the spell's duration expires. Neither side should want to risk their combatants being bull rushed off a cliff. (Unless one side's comprised of kobolds.)

...Instead worry about complete darkness

If the fight's on a plateau in total darkness yet one side can see through the darkness and the other can't, the DM can save some time and just narrate the battle: The side that can't see loses. While the blind side flails about, the side that can see takes their time, pelting the side that can't see with free rocks launched from free slings until either every creature on the blind side dies or dawn. (For high-level characters, this may very well be dawn.)

More to the point, nothing stops a creature from entering a square of empty space from a square of solid ground, but if the creature lacks a fly speed the creature will fall. Prior to this expedition against the Shadow-spewing Gnolls of the Cliffs of Eternal Night, everyone put ranks into the skill Climb, right? Because

It’s practically impossible to catch yourself on a wall while falling. Make a Climb check (DC = wall’s DC + 20) to do so. It’s much easier to catch yourself on a slope (DC = slope’s DC + 10).

Of course, much easier here means not totally impossible. The description of hills terrain, the main source of information on slopes and cliffs, says the DC for scaling a cliff is 15. Thus, by extension, a creature who is blinded on a cliff and moves into a square of not-cliff can make a Climb skill check (DC 25) to catch himself. (It probably goes without saying, but rare is the visually-impaired mountaineer.)

Two Possible House Rules

First, in an attempt to locate foes

A character can make a touch attack with his hands or a weapon into two adjacent squares using a standard action.

A reasonable house rule allows a blind (or sighted!) creature to use the same method to determine if one or two adjacent squares are safe for entry.

Second, movement while blinded costs 2 squares per square traversed, but whether this is due to the care one must take to move carefully, a lack of proprioception, mischievous earth spirits, or something else goes unstated. If the DM interprets this as a creature's natural, reckless sense of being able to navigate wherever he wants being blunted by his sightlessness, a reasonable house rule has a blind creature make a Balance skill check when about to enter a square that the creature could identify as dangerous with another sense (e.g. the heat of lava, the stench of acid, the sound of rushing water, the vertiginousness of a sheer drop). Success means the creature gets an idea of the danger present in the square and may either continue moving into that square, pick a different square into which to move, or stop moving altogether. Failure means the creature enters the dangerous square. The DM and the terrain must dictate this Balance skill check's DC.

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  • \$\begingroup\$ Great answer, but I think the house rules are unnecessary. If the GM really wants a significant chance of falling in the encounter, he can determine certain edge-of-the-cliff squares to have loose rock that could give way, with a Spot check to determine such with the poor visibility acting as a DC modifier. Otherwise, the potential for bull rushes is dangerous enough without adding slapstick 'he ran off a cliff' chances. \$\endgroup\$ – Epiphanis Feb 14 '15 at 7:48

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