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Imagine a situation where PCs meet some magical, evil duplicates of themselves. The duplicates are exact copies of the PCs (stats, equipment, spells, feats and so on). Therefore, I suppose that the chance of winning a battle against these enemies would be 50%. Now, I know that there is not a direct relation between level and CR. What could be the CR of such an encounter (and, consequently, the EXP)? Can it be considered a deadly encounter?

I am also interested in:

  • how the solution to this problem changes with the PCs' level. Is this encounter more dangerous at lower/higher level, or is the difficulty the same?
  • how the solution is stat-dependent and class-dependent? Can it be generalized for every party?

Please note that this is just a theoretical question. As a DM I would introduce some element that the Pcs can exploit in order to ease the encounter, but I would like to know what is the starting point.

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    \$\begingroup\$ Have you tried calculating the PC-doubles' CR using the rules on DMG pp.274-279? What about that didn't work? Given your estimate of 50% chance of success the party clearly "risks defeat" as specified in the Deadly description; I'm not sure how anyone can give a more-detailed answer without knowing your PCs stats. \$\endgroup\$ – nitsua60 Jun 6 '16 at 13:13
  • \$\begingroup\$ I forgot to include one detail that I would like to know, that is, can we generalize this consideration to every party or is it party dependent? I will try to use the rules on the DMG but I am not sure it will work since PCs are usually more complex than monsters and it is difficult to take into account feats, magical items and so on \$\endgroup\$ – firion Jun 6 '16 at 13:20
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    \$\begingroup\$ Having done this sort of encounter in many systems, I would caution that unless your skill within the system are significantly better than your player, the PCs will almost certainly win. They are far more familiar with their characters' abilities and only have to strategize one person, while the DM has to tracks an entire team, each with many options. As a partial answer to your question, this problem will only get worse as they gain levels and therefor abilities and options. \$\endgroup\$ – keithcurtis Jun 6 '16 at 14:15
  • \$\begingroup\$ @keithcurtis a problem at least partially mitigated by having the players either play both the PC and the double, or advising the DM on how to play the double. Something my group does constantly even when we aren't facing our evil twins - we advise each other and our dm on what whoever's turn it is might do with the powers and abilities we know they have to give that character or his team the best possible advantage. \$\endgroup\$ – Alyksandrei Jun 6 '16 at 14:44
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    \$\begingroup\$ Good point, @Alyksandrei. It does require a certain type of player who is willing to "aid their own destruction", so to speak. If an encounter is engineered a certain way, using "double blinds" the GM could even arrange it so the players themselves are unaware which is the genuine and which is the duplicate (the party is awoken out of a sound slumber to discover that everyone has a duplicate). That could be really entertaining. \$\endgroup\$ – keithcurtis Jun 6 '16 at 15:00
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Very roughly, although this is certainly not true for all levels (see Kobold Fight Club), a creature is a medium challenge for four PCs of the same level as its CR. For example, a CR 3 creature is a medium challenge for 4 level 3 PCs. By this metric, a player should be worth about 1/4 of a monster with a CR of their level. We could therefore say that a character's CR is 1/4 of their level.

However, this assumes that the characters are conserving resources in each encounter, as they have several encounters within the 'adventuring day' (pg. 84 DMG). CRs are not balanced for a party that blows every spell slot in one encounter - even supposedly 'deadly' encounters become trivial if that happens.

So, if you play the duplicate characters appropriately (i.e. as if they were conserving resources), then you could reasonably put their CR at 1/4 of their level. To do this, you would just need to use cantrips for the spellcasters, with maybe the occasional spell (depending on their level), and avoid using class features like maneuvers every turn. Essentially, play the duplicates as your players normally play their characters.

However, if you want to play the duplicates to their full potential, chain casting fireballs and the like, then a different approach will be needed. As other answers have suggested, you will need to use the CR rules in the DMG (pg. 274). As an experiment, I decided to calculate the CR of my party of 6 level 6 characters. I assumed that the characters used their most powerful ability for each of the three turns that the damage is averaged over (see pg. 277 DMG for explanation).

Sorcerer:

  • Offensive CR - 6
  • Defensive CR - 1/8
  • FINAL CR - 3

Fighter

  • Offensive CR - 3
  • Defensive CR - 9
  • FINAL CR - 6

Paladin

  • Offensive CR - 3
  • Defensive CR - 4
  • FINAL CR - 4

Wizard

  • Offensive CR - 9
  • Defensive CR - 1/4
  • FINAL CR - 5

Bard

  • Offensive CR - 5
  • Defensive CR - 1/4
  • FINAL CR - 3

Druid

  • Offensive CR - 4
  • Defensive CR - 1/2
  • FINAL CR - 3

Summary

  • Sorcerer - CR 3
  • Barbarian - CR 3
  • Bard - CR 3
  • Paladin - CR 4
  • Wizard - CR 5
  • Fighter - CR 6

I have no idea why this turned up such a range of CRs - perhaps that is the topic of another question. I am almost certain that my calculations are correct. My suspicion is that it is partly due to my group's optimisation, and partly due to some classes having more utility and less simple damage in combat.

What is the Difficulty?

Choosing creatures of equal CR to the ones calculated above in Kobold Fight Club turns up:

  • 10,950 Adjusted XP.
  • The deadly boundary for 6 level 6 characters is 8,400 Adjusted XP.
  • So this encounter would be DEADLY (and very deadly at that).

It's worth noting that 3 CR 1 monsters, and 3 CR 2 monsters (which is equivalent to saying that a character's CR is 1/4 of their level) turns up a 2,950 Adjusted XP, easy fight. That just shows what a difference using each character's most powerful abilities every turn makes.

A Conclusion

I have no idea how this changes across levels, and have no doubt that it will be different for every group, depending on how well optimised for combat the characters are. Nevertheless, it is clear that the encounter will be deadly. Proceed with caution.

It's also worth noting that character's defensive CRs are usually very low (the fighter was an exception, with 20 AC - an AC that isn't even on the CR list). This means that their HP and AC are, on average, much lower than a monster of the equivalent CR. As a result, the encounter should be quick and brutal, with both your characters and the duplicates dropping very fast.

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I have a little experience with this exact situation. A GM once (long ago, in AD&D) threw a duplicate party at a group I was in. We won, but mostly because of dice; two of us were killed out of 5 PCs.

In general, evil duplicates may be more likely to go for immediate kills, a GM group probably "knows" they don't need to hold anything back for a follow-on encounter, and their teamwork may be better than a player group. The result is that an encounter battling opposed "evil" duplicates of the party is likely to go poorly for the player characters if the NPC versions are played intelligently. I'd certainly consider this a "deadly" encounter; my own experience bears it out.

The one potential advantage the players may have is that each of them is (by mid-levels, at least) very familiar with their own character's strengths and weaknesses; there's a real possibility that I might play a character I've been playing for months better than my GM can (the GM has seen that character only from outside for that play time). Whether that can offset the GM advantages above is uncertain -- but it's the players' best chance.

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  • \$\begingroup\$ +1 Deadly for sure. Evil + duplicates makes it an even fight, with the addition that they have absolutely no qualms about murdering you. \$\endgroup\$ – Lino Frank Ciaralli Jun 8 '16 at 5:13
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If you want something definitive, stat up the PCs as if they were monsters

The DMG contains rules for creating "monsters" and evaluating their challenge ratings based (mostly) on:

  • their HP
  • their AC
  • the amount of damage they do in a round
  • their attack bonus

...plus some tweaks for certain special abilities.

There is a section in there about generating NPCs using the same mechanics as PCs, but it essentially boils down to taking all of those stats and putting them through the same process as monsters.

It's kind of laborious, but if you wanted to get a challenge rating for an NPC equivalent of a PC, the "official" way to do it is like that. You can then take those challenge ratings and total them up according to the system for building encounters on an XP budget to figure out how hard the encounter overall is.

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Use NPCs as Guidelines

Take a look at the Monster Manual's Appendix B: Nonplayer Characters (page 342). While not a perfect match, it does give approximate statistics for a wide variety of humanoids with class levels. To extrapolate an actual challenge rating, you'd have to multiply one on-level character by the number of characters you're using, per the encounter difficulty rules.

Example 1

The Knight is an 8 hit die melee fighter. He has basically the normal spread of stats for a character of about that level (if a bit more balanced than I'd expect a PC). Challenge rating 3, 700 XP.

Presuming there are four party members, that's a total of 2,800 XP. The encounter size modifier for 3-6 opponents is 2x, giving 5,600 XP. Divide by 4 to get a per-character XP to comparing that back to the XP Thresholds by Character Level, that's 1,400 XP.

Result: Hard for level 8

Example 2

The Archmage is a level 18 spellcaster with 18 hit die. A quick look at his stats spread confirms (subjective!) that he's about a level 18 wizard. He's listed as CR 12 (8,400 XP).

Presuming there are four party members, that's a total of 33,600 XP. The encounter size modifier for 3-6 opponents is 2x, giving 67,200 XP. Divide by 4 to get a per-character XP to comparing that back to the XP Thresholds by Character Level, that's 16,800 XP.

Result: beyond deadly for a level 20 party.

Summary

Thumbing through a few of the other entries, this method gives results of Hard or Deadly at any level higher than 2. Casters seemed to be on the Deadly side, while non-casters tended toward Hard.

Other Notes

You also have a ton of time to plan in advance; you already know the local geography and presumably their normal marching order. That kind of thing has explicit rules in the DMG for increasing the difficulty further.

Also, your party may or may not know they are in a mirror match. If not, you can use the element of surprise against them. Try an illusion to get the fighter to charge forward into a pit, leaving the squishy sorcerer more vulnerable to the mirror rogue.

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Look up the Aleax monster in AD&D before you run this encounter it will have a lot of useful information. I think it is also in 2nd addition as well.

Zeiss Ikon is dead on correct it is a deadly encounter (or worse) if you run it the way it should go.

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    \$\begingroup\$ External references are great, but please include enough information from the source that the answer can stand on its own. Right now this answer is just "Another answerer said good things, and an off-site resource is helpful in a way I won't explain," and that's not very useful. The Stack's goal of being a self-sufficient repository of solutions means answers need to stand on their own as solutions to the problem. \$\endgroup\$ – BESW Jun 7 '16 at 9:37
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    \$\begingroup\$ Since you strongly believe in your words, let me borrow a few to maybe illustrate something: "Ignore the site's built-in feedback mechanisms all you like. They are providing good information to improve your answers if you care enough about it to invest the effort." The thing is that effort here is expected on the part of every author. If you don't care about your own post enough to make it fit for purpose by meeting minimal quality standards, it's not a very good answer, even if it's right, right? \$\endgroup\$ – SevenSidedDie Jun 7 '16 at 14:10
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    \$\begingroup\$ @40yrGM It is OK to edit your answer to improve it. For example, you could include the fact that this monster is from the Fiend Folio in AD&D 1e, or that it can be found in Planescape material in AD&D 2e. (In 3e Book of exalted deeds) or just provide a link to the Aleax Wikipedia entry. All of these make an answer more useful. If you mouse over the up arrow, the text indicates that an up vote is intended to be for answers that are useful. \$\endgroup\$ – KorvinStarmast Jun 7 '16 at 15:05
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    \$\begingroup\$ So I found the box with my >30yo FF and I'm not clear on how studying the Aleax is helpful. On first blush it's sending a clone of a PC to deal with the PC. But the Aleax also has 100% magic resistance, exponential regeneration, and super-vulnerability to high attack rolls. The impact of that last one is highly AC-dependent. Querent is asking about 5e so they may not have a strong sense of how long combats in 1e tended to last, making the regeneration difficult to evaluate. A few sentences on how you think considering the Aleax would help @firion could make this answer a lot stronger, I think. \$\endgroup\$ – nitsua60 Jun 7 '16 at 17:17
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    \$\begingroup\$ Please share the lessons you 've learned from the Aleax. \$\endgroup\$ – BESW Jun 8 '16 at 2:01

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