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Surprise is elegant for creatures using d6—roll a single die to determine both if and how long the party is surprised, modifying the range on the d6 to determine both likelihood and duration. When encountering creatures that don’t use a d6 to determine surprise, surprise gets … complicated.  Here’s one way to resolve those contradictions.

By default, a 1-2 on a d6 determine surprise, with a duration of up to 2 segments (1 segment on a 1, and 2 segments on a 2) with a potential +1 bonus for being hidden as well as a +1 bonus for being silent.  If  the party is surprised only on 1 in d6, the overall chance and duration of surprise is reduced by 1, e.g., if something would normally surprise 4 in d6, the net result is they will be surprised only on 3 in d6 (DMG, p. 62) for 1-3 segments. You can solve separately for likelihood of surprise and duration of surprise; for this example, one can roll a d6 for surprise, and a d3 for duration— the same probability as rolling one die for to determine both surprise and duration. That can be a useful when determining the duration of surprise for creatures with a surprise range different than a d6.

Never/Always Surprised

Creatures that can never be surprised can be treated as surprised on 0 in 6, and then modified appropriately.  Any time the range of surprise for a group would be reduced to 0 (e.g. , a 0 in d6 chance), the die size is incremented instead (next 1 in d8, then 1 in d10, etc.). Blackmoor uses this for the monk (“At 3rd level monks are surprised only on a roll of 1 in 6, at 5th level only 1 in 8, and at 7th level and above only 1 in 10.”). The same applies in the other direction, if the chance of Surprise is increased to 1-6 in d6, converting the roll to 1-7 in d8, etc.

Determining Surprise Duration

Creatures with a non-d6 likelihood of surprise (e.g, “surprises 7 in 8”) don’t specify the potential duration of surprise, and fairly obviously the greenhag (with a potential 19 in 20 likelihood to surprise) shouldn’t have a potential 19 segments of surprise.

Examining the various monsters, in addition to “hidden” and “silent” other characteristics can increase or decrease surprise: creatures described as particularly silent, being particularly hidden, leaping, keen senses, ultravision, etc. Surprise duration therefore defaults to up to 2 segments, modified by:

• +1 segment for silent opponent
• +1 segment for hidden opponent
• +1 or +2 segments for other extenuating circumstances (extra hidden, extra silent, leaping, etc.)
• -1 segment if harder to surprise (ranger, alertness, vibrations, scent, keen hearing, etc.)
• -1 segment for those “never surprised”

 

To determine the duration of surprise of a non-standard surprise roll (e.g., the greenhag’s surprises on 19 in 20), roll once to determine surprise. Then, independently, roll again to determine the duration using by the creature’s abilities (see Appendix A) to determine the potential duration. If there’s no modifier, the duration range is the default 2 segments—roll d2. If the duration range would be 3 segments, roll d3, etc., with a practical maximum of 5 segments (the longest normal duration of surprise on a d6).

For example, the piercer surprises 19 in 20, but the criterion for the piercer is “indistinguishable from stalactites”, a “hidden” result increasing the potential surprise range by 1m from 2 in 6 to 3 in 6, or 3 segments of potential surprise. Roll a d20 to determine if surprise exists, and a d3 to determine the duration.

Easy! All you need to know is WHY creatures are more likely to surprise (included in monster descriptions, See Appendix A). 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.) listed.

Let’s look at a (if not the) problematic case—greenhag, surprising 5 in 6, but 19 in 20 when invisible:

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 move silently and hide in foliage, and its ability to be invisible. So, presuming the greenhag made a 90% roll, the potential duration of a normal opponent’s surprise should be 5 segments: the base 2 segments, +1 segment for being hidden (in shadows and foliage), +1 segment for being silent, +1 segment for coloration (extenuating)—aligning with the defined non-invisible 5 in 6. In the case of the greenhag, invisibility increases its chance to surprise to 19 in 20, but the potential duration is still 5 in 6 (roll a d5 for potential duration of surprise).

Interactions

In the case where the party is surprised on 1 in d6, the overall chance and potential duration of surprise is reduced by 1. If the party is surprised only on 1 in 6, then if the other side would normally surprise 4 in d6, the net result is the party will be surprised only on 1-3 in d6¹. The potential duration or surprise decreases as well as the chance of surprise.

The Atomie is only surprised 1 in 10. So how do we adjust the atomie’s chance of being surprised (1 in 10) when confronted with a ranger? The ranger surprises on 3 in 6 (1 better than the default 2 in 6). The ranger increases the chance of surprise by 1, so the atomie can be surprised by a ranger on 2 in 10. That 10% increases is not mathematically accurate if you’re working under the assumption that the ranger’s 16.66% increased chance is true for all creatures, but 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 by 1, so up to 2 segments (d2) of surprise.

Since an increased 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.

“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 (you can use 1 on d20 for 1 segment, and 2 on the d20 as 2 segments, or roll surprise separately).

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 due to being “nimble and fast”. Normally they could surprise for up to 2 segments, plus 1 segment for being “nimble and fast”, so capable of surprising for 3 segments (d3). If they’re invisible, they could surprise for up to 4 segments—roll d4 for the duration of surprise.

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 instead of d2.

 

And now, the most extreme example: the 17th level monk vs. the invisible silent hidden greenhag.

• The chance of a 17th level monk being surprised is 2%, and their chance of surprising is 2 in 6. The monk can be surprised for 2 segments, and surprise for 2 segments.
• The chance of an (invisible) greenhag being surprised is 1 in 20, and their chance of surprising is 19 in 20. The greenhag can be surprised for 1 segment, and surprise for 5 segments.

Calculating the chance of the greenhag to be surprised is relatively simple, given the monk’s chance of surprising is the normal 2 in 6. The greenhag is surprised on 1 in 20, for 1 segment.

The crux is comparing the monk’s reduced chance of being surprised (2% or 2 in 100) vs. the greenhag’s increased chance of surprising (19 in 20).  The default is the defender (monk) reduces the range of the attacker (greenhag), so the greenhag would surprise on 17 in 20 (decreasing the range by 2 for the monk’s 2 in 100), for d3 segments (as the potential duration of surprise also decreases by 2).

 

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

Appendix A—Increased chance to surprise

Here’s a list of all the creatures with non-standard surprise rolls, and why. Factors can be: hidden, silent, fast, flying, leaping, special (typically more hidden than hidden or more silent than silent).

MM: Piercer surprise 95%, “indistinguishable from stalactites”, hidden (+1)
FF:Fire Snake surprise 60%, “color matches surroundings”, hidden (+1)
FF:Fog Giant surprise 80%, “blend into fog”, hidden (+1)
FF:Khargra surprise 7 in 8 (5 in 6 against dwarves/gnomes), “leap from wall”, hidden (+1), silent (+1), special (+1)
FF:Meenlock surprise 80%, “moving very silently”, silent (+1), special (+1)
FF:Needleman surprise 75% in undergrowth, “nearly undetectable”,  hidden (+1), silent (+1), special (+1)
FF:Snyad surprise 90%, “totally silent, move with great speed”, silent (+1), fast (+1)
FF/UA:Svirfneblin surprise 90%, “move very quietly”
FF:Xill surprise 90%, “emerge from Ethereal Plane”, hidden (+1), silent (+1)
MM2:Atomie surprise 90%, “nimble and fast”, special (+1)
MM2:Bookworm surprise 95%, “changes reduced to 50% if victim can see invisible things”, hidden (+1), special (+1)
MM2: Greenhag surprise 19 in 20 if invisible, “coloration, 90% move silent and hide in foliage”, hidden (+1), silent (+1), special (+1)
MM2:Grigs surprise 90%, “leaping ability”, leaping (+1) [note, when invisible at will, hidden (+1)]
MM2:Grim surprise 1 in 10, “senses”, special (+1)

Appendix B—Decreased chance to be surprised

FF:Caterwaul surprised 10%, “keen hearing, exceptional sense of smell”
FF/UA:Drow surprised 1 in 8, “move … with graceful speed” and “superior infravision”
FF:Gorilla bear surprised 15%, “excellent hearing, smell, and eyesight”
FF:Kuotoa (Monitor) surprised 1 in 10, “see into the infrared and ultraviolet spectra, and vibrations within 10′ can be felt by these creatures”
FF:Sussurus surprised 5% (15% in windy conditions), “sees and hears though vibrations and disturbances in the air”
FF/UA:Svirfneblin surprised 1 in 12, “keen hearing and smelling abilities”.
MM2: Atomie surprised 10%. “double human hearing, 2x visual range”
MM2/UA:Duergar: surprised 10%
MM2:Greenhag surprised 1 in 20, “highly acute senses, incl. infra and ultravision”
MM2:Grigs 10% surprised, “hearing equal to double human norm, 180′ infravision, ultravision”
MM2:Mimimal (leopard/lion/jaguar/tiger) surprised 1 in 12
MM2:Shator: surprised 10%, “excellent auditory and olfactory senses along with infra and ultravision”
MM2:Swan surprised 1 in 10, “exceptional senses”
UA: Barbarians: Surprised 10% (5% familiar terrain), “hide in natural surroundings”

Worth noting the overall text on minimals says they are 1 harder to surprise, and carnivores are +1 to surprise for those without s special surprise ability noted.

PHB Monk: “At 1st level of experience, a monk is as likely to be surprised as any other character, i.e. 33 1/3%. This chance goes down to 32% at 2nd level, and it thereafter goes down 2% per level, so there is only a 30% chance of surprising a 3rd level monk, 28% chance at 4th level, 26% chance at 5th level, etc.”
And the Blackmoor monk as so clearly the origin: “At 3rd level monks are surprised only on a roll of 1 in 6, at 5th level only 1 in 8, and at 7th level and above only 1 in 10. Note, however, that extremely silent creatures will double surprise possibilities, i.e. halflings, thieves, bugbears, and undead double possibilities.”

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.