The system is this: one can move the normal allowance (whether the DMG's odd-ball hold-over system from OD&D, or the PHB's nicely abstracted system) without penalty.
After that, things get a bit weird and the word "hours" appears in the rules where I think it probably shouldn't and certainly doesn't make sense with the worked example in the text. So, this is my reconstruction of the rule:
- For a forced march of 10-30% over normal movement, there is a multiplier of 1.
- For a forced march of 40-60%, the multiplier is 2.
- For a forced march of 70% or more the multiplier is 3.
So, a character on horseback (24") has a normal movement allowance of 48 miles in the PHB system. Travelling 55 miles in a day counts as 20% forced march and therefore 20% of the following day is lost to rest, taking that day's base movement down to 38 miles.
If the same horse rider moves 90miles in a day, which is 90% over, then 3x90%=270% of a days travel is lost. In other words, the horse must rest for two days and on the third day will only manage 30% of normal movement, or about 15 miles.
All this resting can be ignored and characters can push themselves, and/or their mounts onward beyond 200% of normal movement. At which point each 10% (or part thereof) extra movement carries a 10% chance, cumulative, of killing "beasts of burden" and reduces the hit dice or level of creatures by 1 until they too drop dead.
The loss of hit dice/levels lasts until the character has rested for 8 hours per 10% movement over 200%.
For an example: a 9th level dwarven fighter with 6" movement can travel (using the simpler PHB system) 12 miles in a day. But there is a battle to get to, and so she marches on doing 20 miles instead (70% over normal). She should rest for more than two days, but can not due to the fact that there's a battle on. Forced to fight, she is only able to contribute as a 2nd level fighter.
That's the whole system and some obvious points are left to the DM. For example:
- Can characters rest for some time and recover some levels? I would say yes, so if the dwarven lady was able to rest for a day she would get 4 levels back (at 8hrs per level).
- What counts as a "beast of burden"? I take it to mean normal animals such as horses and mules while, for example, a paladin's warhorse would count as a creature with hit dice. I think that it also covers 0-level characters.
- How does the loss of hit dice/levels interact with hp? Personally, I would tend towards counting each lost level as loss of hit points at random where applicable. Thus, a 12th level fighter losing 2 levels to fatigue would lose only 6hp but our dwarven lady would lose 7d10hp (I would specifically not add CON modifiers to this) and a thief would suffer damage using d6.
- How does lack of rest interact with spell memorization? Actually, I think this is fairly easy to guess; see below.
- What's the effect of fatigue on the rider of a horse or other animal? I've no idea on this one but I'm going to try ½ for now and see.
I'm generally against systematizing any more than is needed - been there, researched the rules for making t-shirts - but fatigue is something that is a common plot device or concern in fiction, and the ability of characters to press on beyond normal limits is something that marks them out as heroes.
Let's get down to generics
Well, this is all nice and good for forced marches; can we use it more generally? I think so. The overall system can be abstracted to:
- There is a limit to what normal people can do in some base time period.
- There are increments over this which anyone can force themselves to perform which incur what I'm going to call "fatigue points".
- Once 10 points have accumulated, beasts of burden have a % chance of dying equal to 10 x fatigue each time a new fatigue point beyond 10 is incurred (so 10% at the 11th point, 20% at 12 etc.).
- For each point over 10, characters subtract their one from their level (and, I think, suffer hp damage as appropriate).
- Each fatigue point up to 10 requires a number of base time periods times 1, 2, or 3 depending on the maximum total number of points a character has reached since they were last at 0.
- Recovery time for points over 10 is 8 hours per point.
- Spell memorization is impossible while fatigue points are above zero.
- Heal removes all fatigue points.
- Cure spells and potions remove one point per d8 of curing done.
- Natural healing does not start until fp=0.
So, a party exploring a dungeon is in a hurry and take no breaks for 10 hours. At this point they have 10 fatigue points and each of these will require 3 hours to remove. But at this point they find the lair of a group of drow and enter combat. Rest is also normally required after a combat but the DM rolls a wandering owlbear attracted by the noise and the party are unable to take that rest and so incur another fatigue point, taking them to 11. Each character who fights the owlbear loses one level worth of hit points and performs at one level below normal.
Blocking up the door of an empty room to rest up, the party finally takes a break. The members who did not fight the owlbear will require 30 hours of rest to get back to 0 fatigue, the ones who did will require an additional 8hrs.
Various spells and magic items may modify these effects is more or less obvious ways.
You mentioned "Nymphs". Where are the nymphs?
No. Appearing: 1
Armour Class: 6 (10+4 for dex)
Move: 12" (see below)
Hit Dice: 3
% in lair: 10%
Treasure Type: 1-4 Jewels 50%, X
No. Of Attacks: 0
Special Attacks: Fascinate, Stunning
Special Defenses: Immune to cold
Magic Resistance: 50% (12)
The ice nymph is a nature spirit of water which has been frozen for a long time (i.e, many years). Their normally warm and beneficent nature has become cold and brittle like the water they were born from and they are generally much more hostile than their MM counterparts.
Additionally, their appearance is different and although lively, and lithe, their flesh has taken on the appearance almost of a corpse with blue lips and pale grey/blue skin. Eyes and hair remain as animated and beautiful as ever, however, and it is unlikely that they could be mistaken for undead at any distance under 30' or so.
They share their sister's ability to use dimension door once per day and can also move over ice or snow as if it were dry, firm soil, leaving not even a foot print.
Their druidic spells are normally: 1st) Faerie fire, Invisibility to animals, Predict weather, Speak with animals; 2nd) Cause light wounds, Chill metal; 3rd) Hold animal, Protection from fire; 4th) Quench fire.
While cold-based attacks have no effect on ice nymphs, fire-based ones do an extra hit point per dice of damage.
The change in their appearance has modified the effect of their beauty. Firstly, looking at one causes only a fascination effect whereby those viewing the nymph must save versus spells (with wisdom bonus/penalty, and an additional +4 for females) or be drawn to pursue the nymph for an embrace.
The nymph may give orders to fascinated males by promising various things but each such promise delayed or broken allows another saving throw with a modifier of +1 for each broken promise. Orders which directly harm the victim or their friends or which normally would be against their alignment will additionally allow a saving throw before the order is carried out. Females can not be so manipulated but are otherwise affected by the nymph's aura.
Should the nymph feel particularly threatened, she may disrobe completely. Doing so has the effect of power word stun on all males within 12" on first laying eyes on her. Females are affected but receive a saving throw.
Ice nymphs are generally haughty and arrogant, even those which are not evil, and are loathe to stoop to disrobing and will only do it very reluctantly and usually to take the opportunity to flee. Naturally, they can not be tracked while in their natural environment.
If someone actually manages to get a hold of an ice nymph, their naked touch will cause 1hp of damage per round to both a warm-blooded holder and to the nymph; increasing at the DM's discretion if the contact is extensive. If the grabber is cold-blooded then double damage is only incurred by them and none to the nymph. Undead may handle the nymph without injury to either.
Ice nymphs are only encountered on ice or snowfields and are usually alone. However, 60% (1-12 on d20) of nymphs have 1d4 frost giants as fascinated bodyguards back in her lair (were their treasure will be added to hers), 15% (13-15) have 2d8 wolves who will be nearby, 10% (16-17) have 1-2 polar bears who can be ridden or harnessed to a sledge for transport, and 5% (18) can summon an ice para-elemental (MMII p98) once per week.
A (rare) friendly encounter with an ice nymph may take the form of a distant voice calling out through a storm to guide a lost party to safety. No matter what the weather, an ice nymph's voice always carries as if it were the dead of the stillest night.
A neutral encounter with an ice nymph will usually involve similar use of their voices to play tricks on a party, the severity of which will depend on the exact reaction roll but will not directly kill them.
A typical hostile encounter with an ice nymph will take place when a lone male is out in deep snow which hampers his movement. If there is more than one man the nymph will probably sow discontent. The victim will then be tempted to follow through the snow, either until they collapse from fatigue or encounter the nymph's companions.
Movement through deep snow is very hard work and movement rates should be reduced to ⅓ of normal, to a minimum of 1". Each half hour of such movement counts as one fatigue point as above (with half an hour as the base time period).
In addition, characters who are dressed inappropriately (and removing bulky and warm clothing will be one of the nymph's first suggestions) will suffer additional penalties: firstly, actually taking rest will not help reduce fatigue unless they can find warmth somehow, and secondly, they will receive an additional fatigue point for each one which would normally be incurred.
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