The surprise mechanics in 1e are a bit counter-intuituve and clunky, which is probably why it underwent some changes in later editions. I think the basic point of confusion comes from the fact that you are NOT rolling to see if you surprise your enemy.
Your surprise roll is better thought of as a "paying attention check" for yourself. Think of it as a saving throw against not paying attention.
You have a base 33% chance of not paying attention. On a d6, if you roll a 1-2, you have failed your paying attention check, so you are vulnerable to surprise. Your "success number" is a 3, because if you roll a 3-6, you passed your attention check, meaning, you were not surprised.
Some monsters surprise on 4 in 6, or better. That means your "success number" is a 5, so on your roll, you have to roll a 5-6 to avoid surprise. A better way of putting it is that they cause a minus 2 in your "attention check" rolls. Your normal "success number" is a 3, but because you are rolling at -2, your "success number" rises to 5.
But what if your surprise roll was already modified, so you are normally only surprised on a 1 in 6? Essentially that means your "attention check" roll is +1. So against the monster that "surprises on 4 in6"/"causes -2 on surprise check rolls", that means you would only be surprised on 3 in 6 by that monster. Your "success number" has risen to 4, because of the -2 and +1.
The number of segments you lose by being surprised is calculated by how far below your "success number" you actually roll. Normally, your "success number" is a 3. But if you roll a 1, you lose two segments to your opponent (3 minus 1) . If you roll a 2, you lose one segment to your opponent (3 minus 2) .
But if your "attention check" number was 5, because the monster was a "surprise on 4 in 6"/"minus 2 on surprise check rolls" monster, that means you could lose up to 4 segments, if you roll a 1. (5 minus 1)
Your individual DEX score can reduce the number of segments you lose to surprise.
DEX 16 = 1 segment bonus
DEX 17 = 2 segment bonus
DEX 18 = 3 segment bonus
Segments lost to surprise are extra important because while you are surprised, you can be attacked once every segment. Think of it as a complete blindside attack, followed by a bunch of easy follow-up blows as you stumble around trying to get your bearings.
We can see that stealthy monsters are particularly dangerous. If you roll a 1 against a "surprise 4 in 6" monster, that means you are subject to 4 attacks in a row, because 5 (your target success roll) minus 1 (your actual roll) equals 4 (the number of segments you lose). You would be subject to 4 straight attack rolls before you could even roll for initiative to counter attack.
The DMG is a bit vague on the this one fine point: can your DEX bonus remove all suprise segments, effectively eliminating your surprise? The DMG states that a low DEX penalty cannot cause you to be surprised, although it could add to the segments you were vulnerable. Think of the big clumsy guy who is stumbling around for longer, vulnerable to more attacks. Just because he is more clumsy doesn't mean he pays less attention than anyone else. His recovery time is slower, but his odds of being surprised are no worse than anyone else.
But what about on the high-DEX side? DEX is said to reduce the number of segments you are surprised by your DEX bonus. But that would seem to imply that your surprise segments can be cut down to zero, and I think that would defeat the whole reality of the surprise. I would say that your DEX adjustment can reduce your lost segments down to 1 from a higher number, but cannot get rid of the surprised penalty altogether.
To allow the DEX bonus to reduce the surprise segments down to zero would be to confuse categories. High DEX allows you to recover more quickly from surprise, but it doesn't prevent you from being surprised in the first place, and if you are surprised, you are surprised, there is no getting around it.
While your teammates are getting knocked about while they stumble around for multiple segments because they weren't paying attention, your lightning reflexes may allow you to spin off the attack, tumble and roll back to your feet to avoid further attacks... But it doesn't prevent your vulnerability to the initial suprise attack in the first place.
Anyway, that seems reasonable to me!
On Megadungeon #3
13 hours ago