Planning for Surprise

Last Updated: 241017
surprise@corwyn.net

Let’s consider a party of adventurers exploring underground. They’re wearing metal armor (audible at 90′), carrying on a conversation, well-lit with continual light (6″ radius, brightness is “very great, being nearly as illuminating as full daylight”). They shove doors open. For most encounters, 33% of the time they’re surprised for 1 or 2 segments. When the monsters know the party is coming, only the party can be surprised!

Learning, the adventurers try to move quietly (at least as quietly as they can). They rely on the available lighting, lanterns (3″ radius), or the dim glow of magic weapons (10′-15′). Now they’re as likely to surprise as to be surprised. Their overall chance of being surprised drops to 25% (9 in 36). Additionally, the party surprises the monsters 25% of the time. They listen at doors (~10% chance, better if it’s a thief), occasionally not being subject to surprise themselves!

Then the party decides to use a scout at least 18″ (and 24″ is better) in advance—far enough ahead to warn the party before the party is within 9″ (9″ being where the party can be heard in metal armor). The scout, a dwarven thief attempting to move silently, sometimes increases the base chance of surprise to 3 in 6. When moving silently, he’s twice as likely to surprise as be surprised! With surprise, the dwarf can retreat to the party to warn them, or choose engage with surprise (with a back stab multiplier), given the party will (hopefully) arrive the next round when hearing sounds of combat.

Given the dwarf’s success, the party halfling thief asks to assume that role (it looks fun!). When not in metal armor and 9″ in advance, the halfing surprises 4 in 6 (as would an elven compatriot in that role). The halfling, now surprised only 14% of the time, surprises monsters 53% (19 in 36) of the time.

  • Moving Silently increases the chance of surprise. If the player of the thief makes their Charisma roll, they might convince the DM that Move Silently (or invisibility) is additive to the elven and halfing base of 4 in 6, creating a 5 in 6 chance of surprise. Good luck with that.
  • A Dexterity of 16 would reduce the halfling’s segments of surprise by 1, then only affected by surprise < 1% (2 in 36) of the time for only 1 segment! If our thief has a Dexterity of 17 or 18, under these conditions he cannot be affected by a creature with a normal 2 in 6 chance of surprise, while able to act with surprise 53% (19 in 36) of the time.

Still, our halfling occasionally encounters creatures with a better than usual chance of surprise. So he’s replaced by the party’s human ranger. The ranger has no restriction for metal armor, so typically a lower AC and higher overall damage. The ranger’s base chance of surprise is 3 in 6, but the ranger also reduces his chance of being surprised by 1 (typically to 1 in 6). For a normal encounter, the ranger will be surprised < 1% (3 in 36) of the time, yet surprise opponents 42% (15 in 36) of the time!

  • With a Dexterity of 16 or better, the ranger will never be affected by surprise in this scenario.

Some creatures surprise better than 2 in 6. When our ranger comes across a spider (chance of surprise 5 in 6, reduced to 4 in 6 as a ranger), odds are someone is going to be surprised! The ranger will be surprised 47% (17 in 36) of the time, while the spider will be surprised 25% (9 in 36) of the time.

  • Even with a Dexterity of 16, the ranger is still surprised 31% (11 in 36) of the time. The ranger requires a Dexterity of 17 to swing the odds in his favor (17% (6 in 36) of the time).

Not good enough odds requires the party to search for a replacement ranger. The replacement ranger is an elf, surprising 4 in 6, but as a ranger also reduces the chance of being surprised. When encountering yet another spider there are even odds of surprise—28 of 36 encounters will have surprise of one sort or the other!

  • A Dexterity of 16 changes those odds to 31% (11 in 36) that the ranger will surprise, yet still surprised 39% (14 in 36) by the spider. A Dexterity of 17 would reduce that to an only 14% (5 in 36) chance of being surprised, and an 18 makes that only < 1% (2 in 36)!

Pairing up our elven ranger with our halfling thief, their chances stay the same—now we have two on recon; the ranger’s reducing the likelihood of surprise applies to his companions as well.

There’s value in handing bracers of defense to those on recon (so no metal armor). As for a party of elves and halfings not in metal armor? They are more likely to have a high Dexterity (as both races gain a +1 Dexterity bonus). They can range through a dungeon with impunity (surprising 53% (19 in 36) of the time), regularly surprising their opponents while being virtually unaffected in return.

With some intent, planning, and party composition, a party can greatly decrease the chances of being surprised (and increase their own chance), going to far as to be able to virtually eliminate the chances of being surprised.

Excel Spreadsheet for calculating surprise probabilities

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Author: Rick

A DM for *mumble* years, I've been playing AD&D since junior high. I've currently got two separate campaigns running, both in Mystara. I've been told when they handed out hobbies, I stood in the short lines. I actively cycle tour, kayak, play board games, read, develop home automation software, play Stars!, volunteer with the International and National American Red Cross, and work on a never-ending stream of home repairs. In my wake I've left paintball, medieval full-contact combat (SCA), computer gaming, Heroclix, and kite construction.

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