Last Updated: 240216..240219

Surprise is elegant for creatures using d6—roll a single die to determine both if and how long the party is surprised. When encountering creatures that don’t use a d6, surprise gets  … complicated.  But what if you look at surprise not by dice, but by segments? The non-standard likelihood of surprise doesn’t usually specify duration of surprise. So what if you decouple those two answers, and use one roll to determine surprise, and a second roll to determine surprise duration?

Default surprise is 1/3 of the time (1-2 on a d6), with a duration of up to 2 segments ( 1 segment on a 1, and 2 segments on a 2). Let’s solve separately for the two cases: likelihood of surprise, and duration of surprise. A d6 for surprise, and a d2 for duration— the same as rolling one die for surprise and duration.

Determining Surprise Duration

By default, the base duration of surprise is 1 or 2 segments. If the opponent is invisible or hidden, surprise can potentially last one segment longer than otherwise. If their opponent is silent, surprise can last for one segment longer than otherwise. Additionally, a number of things that can increase surprise by an additional segment or two: creatures described as particularly silent, being particularly hidden, etc.

In the case where the party is surprised on 1 in d6, the overall chance to be surprised is reduced by 1. In this case if the other side would normally surprise 5 in d6, the net result is the party will be surprised only on 4 in d6 (DMG, p. 62). This of course affects duration as well as overall likelihood of surprise.

So surprise can normally last up to 2 segments (1 segment for creatures that are 1 in x), modified by:

• +1 segment for silent
• +1 segment for hidden
• +1 or +2 segments for other extenuating circumstances.
• -1 segment if harder to surprise (ranger, alertness, vibrations, scent, etc.)

Determine whether there is surprise. Then, independently, roll for duration based on the possible range. If the range is 2 segments, roll d2. If the range is 3 segments, roll d3, etc., with a practical maximum of 6 segments. This works for all of the creatures that surprise on a d6.

So let’s look at a (if not the) problematic case—the greenhag:

Because of their coloration and the fact that they are 90% likely to be able to move silently and hide in foliage (even of the underwater sort), greenhags surprise 5 in 6 (19 in 20 if attacking from invisible state).

The greenhag has several things going for it: its coloration, its 90% ability to hide in shadows and hide in foliage, and its ability to be invisible. So, presuming it’s both invisible and made a 90% roll, the duration of a normal opponent’s surprise could be 6 segments: 2 segments, +1 segment for being hidden (in shadows and foliage), +1 segment for being silent, +1 segment for coloration (extenuating), and +1 segment if the greenhag is invisible, opponents will be surprised on a 19 in 20, and be surprised for up to 6 segments (roll a d6 for potential duration of surprise).

Easy! All you need to know is WHY creatures are more likely to surprise (included in monster descriptions). Roll their likelihood to surprise, and you can deal with the surprise duration as a separate roll.

“Because of their highly acute senses, including infra- and ultravision, [greenhags] are surprised only 1 in 20.” Roll a d20. If the greenhag is surprised, it is surprised for 1 segment. If the greenhag’s opponent is invisible, the greenhag can be surprised for up to 2 segments (roll a d2 to determine surprise duration), etc.

Determine Surprise Likelihood

The above resolves for how long a creature or party could be surprised IF they’re surprised. And, in the simple case, we’ve determined the likelihood of surprise: 2 in 6, unless the creature is less likely to be surprised. The greenhag is only surprised 1 in 20. The Atomie is only surprised 1 in 10. If they’re surprised, determine surprise duration separately—one segment unless their opponent is hidden, silent, invisible, etc.

That works for all the creatures that don’t use a d6 for surprise. Whether it’s a percentage, or a different die (d8, d10), you can determine whether they’re surprised, and determine the duration depending on the conditions of surprise (hidden, invisible, etc.)

Interactions

In the case where the party is surprised on 1 in d6, the overall chance to be surprised is reduced by 1. In this case if the other side would normally surprise 5 in d6, the net result is the party will be surprised only on 1-4 in d6¹. This is an important distinction because the chance of being surprised decreases, in addition to the potential duration.

So how do we adjust the atomie’s chance of being surprised when confronted with a ranger? The ranger surprises on 3 in 6, which is 1 better than the default 2 in 6. In this case, apply the same math we used for duration:

The atomie is surprised 1 in 10. The ranger increases the chance of surprise by 1. The atomie can be surprised by a ranger on 2 in 10. That is not exactly accurate mathematically if you’re working under the assumption that the ranger’s 16.66% increased chance is true for all creatures. Consider that where the party is surprised on 1 in d6, the overall chance to be surprised is reduced by 1 on the die, not 16.67%. The atomie is not a normal creature! Why, by default, assume that the ranger’s increased chance is 16.67%? It makes sense that the ranger is slightly less effective at surprising the hard-to-surprise atomie. If the atomie is surprised, the ranger also increases the potential duration, so up to 2 segments (d2) of surprise.

Since the chance to surprise is always additive (typically +1 segment), you can readily adjust the chance to be surprised by adding 1 to the range, whether it’s d8, d12, or d20. If the chance to be surprised is in percentages, convert it to a standard die (10% is d10, etc.) to make the modification.

Creatures that can never be surprised can be treated as surprised on 0 in 6, and then modified appropriately.

Examples:

A piercer surprises 95% of the time (19 on d20). Roll % for surprise. Surprise could last up to 3 segments (the standard 2 segments, +1 for being hidden (“indistinguishable from stalactites”).

The atomie surprises 90% of the time. Normally they could surprise for up to 2 segments. If they’re invisible, they could surprise for up to 3 segments—roll d3.

A 5th level monk is surprised 26% of the time. Roll % for surprise. Surprise could normally last up to 2 segments (the standard), but if their opponent is silent and invisible it could be up to 4 segments—roll d4.

 

So there you have it. When in question roll (and modify) the chance to surprise and the duration of surprise separately.

References

¹DMG, p. 62.

“Example: Party A is surprised only on a roll of 1, but party B surprises on 5 in 6 (d6, 1-5) due to its nature or the particular set of circumstances which the DM has noted are applicable to this encounter. The favorable factor normally accruing to party A is 1, i.e., parties of this sort ore normally surprised on 1 or 2, but this party is surprised only on a 1 – therefore they have an additional 1 in 6 to their favor (and not a 50% better chance). Party B will surprise them on 5 in 6 less 1 in 6, or 4 in 6. Assume A rolls a 4, so it is surprised for 4 segments unless B rolls a 1, in which case A party’s inactive period will be only 3 segments, or if B rolls a 2, in which case surprise will lost for only 2 segments (4-1 = 3,4-2 = 2). ”

Dragon Magazine, Issue 133, “Surprise: Determining who gets the drop on whom”, May 1988.