Part of my AD&D House Rules, Part 2 - Stealth and Surprise

Sometimes we internalize years of rules that it becomes hard to explain on the fly. Recently, there's been a lot of questions around the idea of surprise and stealth. I've taken the time to spell it out for how I rule things citing the AD&D rules where at all possible, explaining reasons when not.

There's not a lot in hard, fast, or concise rules in AD&D but some can certainly be extrapolated. A few items are because I wanted to codify how some scenarios played out for transparency.

Surprise

Surprise (meaning to engage in combat in any way) standards apply. Rushing into a room of monsters where they are unaware calls for surprise. 

• Surprise is not stealth. 
• Surprise circumvents stealth when the attack is initiated.
• Surprise can only be attempted outside of hearing range.
• Random Encounter surprise distance will be 1"-3". YMMV for planted encounters.
• Surprise checks starts the moment the surprised party could be aware of the opposition (line and direction of sight, as well as sound).
• Undetected (stealth, invisible) Surprise Chance is 4-in-6 (PHB103, MM39 "Elves")

Stealth

Sneaking with Move Silently

If you can hear an alert monster, they will hear you if you fail to move silently within certain distance.

• All distances are as the Ghost flies.
• Provided that they are intact and the walls are of standard thickness, doors HALVE the ranges below (my rule).
• Loud noises DOUBLE the range (my rule).  (sounds of battle, chopping down a door, chipping away a brick wall)
• Unalert HALVE the ranges. (distracted, sleeping, eating, bickering over dice, etc)
• Overwhelming noise halve the ranges. (active mining, a loud thunderstorm with a downpour of rain, etc)
• Rangers enjoy 60' for metal armor due to facilitating their surprise. 
• The below distances are based on a pursuer listening for someone, which is equivalent to monsters being alert (DMG68).
• Given the above, characters in metal armor moving cautiously (1/10th movement, PHB102) can be heard at 60 feet, 30 feet for rangers.
• The below section under NOISE is extrapolated to loud, normal, and quiet noises(my rules). 
• Spellcasting and normal conversation is normal (my rule).
• Loud noise always puts monsters on alert at best, gets them to come investigate at worst.
• This applies to monsters.
• Detect Noise, spending the time and a successful role could give the approximate distance.

Quick examples

• A solid room (walls a few feet thick, intact, enclosed) with solid closed doors, but loud noises would balance out. (example: hammering away at a brick wall in a closed chamber is monster-audible for 90 feet. Open the door, because you're a prankster who hates your friends, and it's 180 feet).
• A solid room with solid closed doors in the middle of an active slave-mine where pick axes and mine carts and making a ton of noise will bring the range of two fighters in a battle with two gnolls in half twice (90 to 45 to 22.5 feet). The guard station 40 feet away won't hear it, unless someone swings open the door.
• An open door in a giant quiet crypt chamber, while adventures are chipping off mineral build up, with hammers and chisels, off of a big sarcophagus is going to be 180'.
• A magic-user casts invisibility in a closed room can be heard 30 feet away.

Really Drawn Out Examples

Situation A: A fight breaks out with oil bottles shattering, zombies being lit on fire and the frenzied snarling zombies ripping vines out of the wall to free themselves (this is sounds of battle). Small monsters to the north are busy feeding a big monster. I'm going to call shattering glass alone on the same level as loud, which means they are now on alert from 90'.

Red guy heard zombie violence only 55 - 60 feet away.

Situation B: A bunch of noise just came from the south. The monsters are in a depression in a big cavern excitedly communicating with each other over what they just heard. One of the monsters towers up over the edge giving it only a quick roll of the eyes or turn of the head to see what made the noise. 

Red monster is alert.

A thief needs to get about 10 feet closer than 30 feet to look at the edge to get the information he is after because of line of sight. He makes his intentions known and says he is going to move silently. A character in leather armor (so relatively quiet) makes a move silently roll as they creep forward to look over the edge of the pit.

Damn it, geometry!

He is level 1 (base 15%) with a dex of 18 (+10%), so 25%. He rolls a 40 and fails the roll. Relatively quiet is 30', therefore his noise is heard. He's been spotted.

Situation C: Situation A just occurred. The party has no thief. Just fighters in metal armor, all human too. Dirty, boring, filthy humans. One of the fighters decides to peer around the corner after the zombie-smash bottle fiasco. Unfortunately, the smashing bottles and frenzied zombies had put the monsters on alert already. They can hear his loud-ass platemail creaking a significant distance away.

Situation D - Sneaking with Move Silently and Hide in Shadows: Similar to situation B. A bunch of noise just came from the south. The monsters to the north are on alert. A thief needs to get about 10 feet closer than 30 feet to look at the edge to get the information he is after because of line of sight. He makes his intentions known and says he is going to move silently and hide in shadows, sticking to the sides to try to take advantage of the shadowy folds. 

A character in leather armor (so relatively quiet) makes a move silently roll as they creep forward to look over the edge of the pit. He also makes a hide in shadows roll (to blend into dark areas, conceal body heat behind rocks, flattening himself, etc). His chances are respectfully 25% and 20%. 

Outcome 1:

He rolls a 34 and a 14. As established above, he makes noise creeping forward, however with his very slow and careful movements to stick to the shadows, he remains hidden.

The noise is heard 30' away, but they can't see what made it. The thief is safe for this very second, but the monsters might come looking for him. Does he try to slink off? Or is going to wait like shadowy death at stab one of the fuckers when it comes looking?

Outcome 2:

He rolls a 30 and a 68. Shit. He's been heard AND spotted. They heard the noise, they looked, and he wasn't as well hidden as he thought.

Situation E: The elf has silently peered around the corner. He successfully creeps past the passageway north to rejoin his party, a paladin in plate-mail and a magic-user. He gives them the situation: two ghouls, at least, are in the cavern to the north chewing on femurs. 

The paladin has no fear, only distain, for the creatures and rushes to attack, hoping to gain surprise. She has to travel 40 feet to the passage way. As the ghost flies, she is 40 feet away from the nearest ghoul. She inches (1/10th of her movement) her way 10 feet closer, because she's one hell of a meta player. The ghouls are unalert so the 60 feet goes to 30 feet. She would heard if she were attempting stealth further. However, she is attempting to engage in combat and this calls for surprise.

The ghouls roll a 2 and they are surprised for 2 segments as she starts charging. Her movement rate is 9, which means she's moving 36 feet in her charge, which gets her to the passage to the ghouls chamber as she continues her charge.

Situation F: The elf had listened to the door and heard the sounds of something slurping merrow out of bone. He reports this back to the party who has been careful up to this point. The ghouls inside are not alert and behind a solid closed door in a solid room. This causes their hearing to be quartered. The paladin in platemail (creeping, so 60 feet) can actually only be heard 15 feet away. 

The party decides to burst through the door and kill what's on the other side, hoping for surprise. They quietly get into position, the thief ready to open the door so the paladin can rush in.

Surprise is checked at this point, fairly.

Situation G: The elf assassin, peering around a statue, sees a ghoul up ahead. 40', sniffing the air and peering around in the hallway.

The assassin decides he's going to sneak but not hide, since hiding would dramatically reduce his move speed. He is concerned that this ghoul will run off to join it's pack in the caverns to the west. His move silently is 30%.

Outcome 1: He passes his move silently check, closes the distance and the Ghoul is surprised on a 1-4 out of 6.

Outcome 2: He fails his move silently check, but has committed to closing the distance for the strike. If the ghoul is alert, it hears him 30 feet away. The noise, even if minor, was picked up on the alert ghoul 30 feet way. Surprise is checked normally (1-2 on a d6) and even with the surprise success, the thief still has to cross 30 feet.

Had the ghoul not been alert (eating a rat instead of actively sniffing the air and looking around), he could get within 15 feet moving relatively quiet which would make his success on surprising require less distance to travel (and therefore more time for attacks).

Situation H: Replace the above Elf Assassin with a Half-elf Ranger.

The ranger is in metal armor, but moving cautiously and 20 feet more south (30 feet hearing on the alert ghoul), peering behind the statue. He moves up carefully to try to get the jump on it. He knows he's gonna have to go for it immediately.

Outcome 1: He surprises on a 1-3, meaning at a movement rate of 9, he's getting to 18 feet in 1 segment, 36 feet in 2 segments (giving him his charge attack), 54 feet in 3.

Outcome 2: There's a door in between them, but he knows the ghoul is there. 

A surprise roll of 2 (36 feet) or 3 will suffice for getting the charge in on a surprise.