Encountering Difficulties?

I've read many discussions on how lots of low level opponents compare to a single high level opponent. For example recently at Roles, Rules & Rolls, in Overkill and Monster Experience Roger discusses eight 1HD monsters compared to a single 8HD monster and says they are much easier to kill - he calls it overkill.

"a group in control of tactics is going to have a much easier time dealing with 8 x 1 HD orcs than 1 x 8 HD giant, for any number of game mechanical reasons."

In contrast at Hack & Slash, Courtney Campbell complains that in D&D5e

"An encounter with 12 CR 1/2 Kobolds is more threatening than an adult dragon."

So how does the number of monsters and PCs affect the difficulty of the encounter?

Let us imagine 1 monster versus one PC and that it takes the PC three rounds on average to kill the monster. Then, on average, the monster gets in about 2.5 attacks before it is killed.

What about 3 monsters versus one PC? It takes nine rounds to kill all the monsters on average, but initially there are 3 attacking you until you kill one, then 2, then 1. So on average each round you would imagine you get hit 3, 3, 2.5, 2, 2, 1.5, 1, 1, 0.5 times, or 16.5 in total; about 6.6 times as many times as one monster would (roughly a triangular number progression).

When I ran a combat simulation it actually came out slightly higher than this – the strength was increased by the square of the numbers. I previously measured strength of opponents via a combat simulation to be how many of them you could kill on average (facing one after another) before they kill you. When you repeat this with multiple opponents at a time, the number you can kill is roughly divided by the number of opponents.

So the effect of changing numbers in a combat is to alter the difficulty of the combat by the square of this ratio: halving the number of PCs makes it twice as hard, treble the number of monsters and you make it nine times as hard, double the number on both sides and the difficulty remains the same.

We have previously seen that an 8HD monster is about 25 times as powerful as a one HD monster, this is the same as meeting twenty five 1HD monsters one after another. But this rule shows that the group of eight 1HD monsters as a single group is 36 times as powerful as a single 1HD opponent. Hence eight 1HD monsters in a single group is a more serious challenge than the single 8HD monster - the opposite of what you might expect!

Similarly the 12 kobolds in reality may be poorly armed and armoured, and you may be able to use an area attack such as fireball, and there are odd rules about multiple attacks specifically against 1HD opponents, but they certainly have the potential to be lethal against a lone high level fighter.

This corroborates with my experience - single high level foes are surprisingly easy to kill when they are outnumbered, there is a lot of truth in the adage “safety in numbers”. To make a challenge from a high level monster you need to give them something more than just better chance to hit and more hit points.

So how can we design our game / scenario / monster to mitigate against these issues - stopping single high level monsters from being a push over, and keeping hordes of low level monsters a challenge, and giving appropriate XP for both types?

Monster Tactics
As Roger points out, tactics can change the difficulty of a combat markedly. We don't want our mass of low HD monsters to be a push over, a meaningless slog fest, or easy XP. Monsters are not mindless zombies (unless, of course, they are mindless zombies!) - if it is obvious that they are individually outclassed but have greater numbers they should not allow themselves to be bottlenecked. If on the attack, they should look to ambush or sneak around behind and surround the PCs. If on the defensive they should retreat until they have bottlenecked the PCs.
Similarly, single monsters should bottleneck opponents, it's especially good for them to block the exit! If in danger of being surrounded, they should retreat so their back is to the wall.

Appropriate XP
In White Box D&D XP was linear with HD (100XP per HD) leading to the "too much XP for low level monsters" problem discussed above. In Greyhawk this was replaced with the familiar roughly exponential system, but this means that in Greyhawk you get 65 times as much XP for the 8HD monster, which is over generous (or, rather, the XP for 1HD is stingy). Killing a single high level monster gives you  inordinate amounts of XP, so should probably be reduced down to only 25 times as much (in line with how hard they are). Here is a suggested revised XP table:

Level	White Box XP	Greyhawk XP	Revised XP
1	100	10	25
2	200	20	50
3	300	35	100
4	400	75	175
5	500	175	250
6	600	275	350
7	700	450	450
8	800	650	600
9	900	750	750
10	1000	900	900

This is a balancing act. In Explore I've reduced the amount of fighting required to progress significantly, hence there is a real danger of massive improvement from a single encounter, so I reduce the XP even further (to only 12 times as much XP).

Extra lethality
Instead of making the monster harder to kill or harder to hit, make the attacks more lethal.
In D&D you find that Giants (for example) get extra dice damage, which makes them tougher, but most of all that makes them capable of killing you, and a single blow becomes potentially fatal. For example, if a party of PCs meets a large number of Orcs or a single Giant, the Orcs may do more damage in a combat, but they're liable to spread that around as a few injuries which are easily healed, whereas the giant may take out one of the party with a single hit.
Poison or paralysation or other nasty attacks (level drain anyone?) also make the lone monster “feel” tougher as the consequences of being hit.

Individual Initiative
My experience with side based initiative is that if you win initiative against a single foe you can often wipe it out without it making a single attack, which is a massive anti-climax. So I’d recommend individual initiative in such situations, even if you don’t use my initiative system the rest of the time.

Henchmen
Give the monster henchmen who soak up the PCs: E.g. this Giant has a bunch of annoying Orc lackeys who engage the PCs in combat and stop them all attacking the Giant at once.

Engaged In Combat
Note that henchmen work best if you insist on people being engaged in combat with an opponent – if someone attacks you, you have to engage them in combat else they get a rear attack bonus, and you can only attack someone you’re engaged in combat with. In general this also helps stop people all ganging up against a single opponent.

Area Attacks for Monsters
Dragons get their toughness from their area attack. A bunch of kobolds attacking a Dragon would be wiped out. Area attacks stop people emasculating your nasty monster purely by virtue of numbers. I'd either restrict such monsters to always be on their own, or to disallow multiple area attacks of the same type against an opponent in a round  - e.g. if two dragons breathe on you in the same round then you only save against one.

No Area Attacks for PCs
On the other hand, Area Attacks for PCs are very problematic - they turn low level monsters into easy XP, and they can wipe out all the henchmen - which is one reason why I've removed mass spell attacks from Explore. Fireball comes from Chainmail where it makes sense, but it's massively overpowered in D&D. Gygax nerfed it in OD&D firstly so that you have feet indoors versus yards outdoor (which is random), and secondly so that it blows back and kills the PCs, which is not the sort of game I like to play - in fact I think this sort of nerfing is always a bad idea (magic user armour restrictions and super low hit points, demihumans level limits).
You can't really remove Fireball from D&D though and still call it D&D...

Multiple Attacks

Give the monster multiple attacks against different opponents.
E.g. allow a giant to attack up to three man-sized opponents so long as they’re not behind him (one swing at several opponents) – one attack roll for each of them. A Dragon could do the same with its tail and attack everyone behind it. Using engagement rules again, this advantage only comes into play when the giants are outnumbered.