Advantage to a literal "Army of Darkness"

Question

If an army composed entirely of creatures that have darkvision leveraged their ability by attacking only a night, how significant would their advantage be?

Under normal circumstances, darkvision is an enemy ability that can be worked-around fairly trivially with low-level magic or proper planning when dealing with it in a typical adventure encounter. However, I believe that would change on the scale of a full-blown war - the question is, by how much?

If a force (a hundred or more) of creatures that all could see in the dark (undead, demons/devils, etc) were to besiege an army that all could not (humans) during pitch-black night (no visible moon) in full warfare, how much more of a difference would the ability make? Would the advantage be much more difficult to counter? (i.e. unable to generate sufficient light in large enough areas to negate the advantage) Would the defending side be at dim light or blinded penalties at all times? How substantially would it raise the approximate challenge rating of the monsters? What logical counter-measures might the disadvantaged defending side take? Would these creatures be ludicrously more difficult than they would be normally, or would the result of a night-time battle be largely only stylistic.

Planning is being done for a 3.5 E6 campaign, meaning that low-power, low-magic rules are in effect - no spells over 3rd spell-level can be brought to bear in this scenario by either side, though spell-like abilities that replicate these powerful spells, such as those possessed by evil outsiders, are still existent.

Answer 1

Darkvision has a really limiting clause in its workings that cripple its effectiveness in field battles — Maximum Distance.

While, during the day, almost any creature can see really far away, darkvision only works in the immediate proximity — the typical darkvision range for undead is 60ft, which is almost nothing on a battlefield (roughly 18 meters). Drow can see a bit farther, their darkvision going up to 120ft.

So, while darkvision is really great for creatures that live in pitch black darkness inside dungeons, it can't really benefit an average army.

To give an example, some real-world castle walls topped 80ft, without any use of magic. That means that, if an undead soldier stood still right at the foot of a very high wall, it couldn't even see where the wall ended!

Also, siege engines would become useless, since you can't even aim then. How would you hit something that is a quarter of a mile away, if you can't even see beyond 60 feet?

Of course, one could argue that you can fire a trebuchet at a city at night using its lights as a target. This can be an advantage, since creatures with darkvision don't need light to operate the siege equipment (thanks to @KRyan for pointing this out).

To be really effective, darkvision users must rely on guerrilla tactics, ambushes and similar strategies. In open battlefields, this advantage is almost null.

Answer 2

Logically, Not much

Speaking only to the narrative, let us just think about the human army for a moment. They are at war, and they know it. You cannot move a large army to siege a fortification - and siege the fortification - in a single night, so they would know the army is coming. They would also, presumably, know that the creatures they are at war with can see in total darkness. There would be nothing stopping them from creating as much light as they can to take that advantage away. Mechanically speaking, some of those creatures are even at a penalty in bright light, so a large army of darkvision monsters could easily backfire.

Not so much for a small, efficient, tactical strike team. 'Special ops' Drows would be at an extreme advantage, since they CAN approach without being noticed in the days before the attack.

Answer 3

An entire army of such creatures wouldn't be at such a great advantage. Probably the best case scenario for the darkvisioned army is encountering the opposing force in the middle of an empty field at night under peak darkness conditions. Even so, it could be assumed that the opposing force, since they need light to see, would probably keep torches or something handy to see at night. One torch for every couple of soldiers would be enough to keep the daylight-visioned soldiers effective in hand-to-hand combat. Daylight-visioned soldiers would of course not be able to see very far into the conflict, but neither would the darkvisioned soldiers since darkvision often comes with the drawback of a maximum range. In this best-case scenario, though, the darkvisioned soldiers would definitely have the advantage initially, especially if the daylight-visioned soldiers are largely asleep.

The darkvisioned soldiers would probably be the most effective, though, as a special team in a larger army. This would give them the benefit of being able to make quick strikes against vulnerable enemies without getting into a conflict of a scale that they can't see. They would likely make great guerrilla fighters (assuming smaller groups) since their enemy is unsuspecting and they will be able to play to their strengths while avoiding their weaknesses.

Answer 4

So, since we're talking armies, we're also talking about large scale logistics.

Night Spies & Recon

To be sure, large armies are pretty easy to spot at night - campfires alone tell you a lot. The advantage for night vision troops is that their spies can move faster at night - when you have full sight, you can walk/run at full speed without worrying about tripping over stuff or falling down an embankment, etc.

The night vision troops also can find the other side's sentries or scouts much easier as well - either they use light, in which case you can see them from afar, or they don't use light, in which case, they have the sight disadvantage.

Movement

Well, here's a big advantage. Moving at night is usually a bad idea because the troops can't see that well, so everything is slowed down. When that's not an issue, night movement becomes a pretty great option. Assuming these troops aren't penalized in daylight, you now can choose when you want to move (day, night) and take the biggest advantage of the timing, which the daylight troops cannot really do.

Ranged Combat

Being able to more accurately shoot at range, in the dark, than the opposing side is a huge advantage. You have a range advantage the other side can't match because you can see further and target correctly.

I think overall, there's a lot of advantages to being able to see at night (see, modern warfare and night vision goggles), probably to the point of bumping things CR+1. If the monsters are especially smart with their tactics, maybe even CR+2.

Answer 5

At the time I'm writing this there are three answers that basically say "not much" and one "three advantages not to be dismissed". So I will try to give you a scalable approach. Most likely your players will tend toward the "not much" attitude and try to employ countermeasures or OOC-reasoning along the lines of those answers.

The point is: What do you want to achieve?

The approach

As others have noticed, it's hard to hide an army. But how does this apply to an army of darkness? If you want the defenders to be prepared, have them notice the army at any convenient time before they reach the fortications. However if the defender shall have little warning there are some points to consider. What if there are hiding places along the way? If your army doesn't need to breath even a lake would hide a complete army and get away from anyone trying to spy on them. Thick forests work out, if spying defenders are caught. Does the army of darkness tire? Maybe they can march off-road where no one would suspect an army to make such progress. Marching all day and night doubles the speed and should surprise any defender not used to fighting of such an opponent. (Reality's history: The french dismissed recon info as "the germans could never have made such progress in the short amount of time" at the beginning of WWII)

Realistic siege

Sieges are boring. I mean deadly boring. That's the point. People outside the wall wait for people inside the wall to die (of hunger, not boredom but that's not any more interesting). But how does this change, if the people inside the wall know straight from the beginning that their foe can outlast them, because they are undead/demons that do not have to eat? The besieged will have to put up a fight early on. So you can have a short siege phase if you want to and then start a prepared counter attack.

Field Battle

Any counterattack of the defenders will happen at day. This is an unusual thing, because the attackers are probably outnumbering the defenders (why else would they start an attack?). Usually one might expect some stealthy counterattacks by defenders at night. In your case this wouldn't work out good.

Biggest Advantage (not mentioned by now)

The answers so far have addressed tactical/analytical advantages and some of them read like "How my character would counter it". But lets face it: People at war are not always analytical. One might think that imminent risk of death is a good motivation to fight "all out". In fact, people panic at certain points. The famous book "Art of War" by Sun Tzu is only some few pages long but dedicates a whole chapter on how to get your army into a position so that your soldier actually will fight (instead of fleeing or just defending themselves). Sun Tzu advises to get your own army cornered (in a tactical advantegous position)!

That long preface is meant to stress this point: Fear. The best weapon an army of darkness has is the deeply set fear of darkness in most people. RPG heroes are certainly not your average soldier, which can be good enough to reason why they often act fearless. Well would they be heroes otherwise? But an army gathers together whoever is able to hold a sword, spear or even fork. What will the average villager do, if he is forced to fight against an enemy that's lurking in pitch black night? Most likely he will huddle around fires or torches. Every torch bearer is one fighter less (average townspeople can't fight two handed with a bruning torch). And huddled groups are fair game for ranged attacks, which don't suffer from darkness as their target is lit up.

Plus, a single soldier does not have to look far in melee combat. What is it to you (undead or not) if someone 40 ft away needs your help , if there are 20 enemies between you? 60ft dark vision is a great advantage in melee. Since you defined there's fortifications the defenders can choose when to leave it and they will most likely make a counterattack at day.