D&D 5E How Deadly is a "deadly" encounter, on average?

[Trevalon Moonleirion]

Aha! That seems to make a bit more sense. We'll find out tomorrow for certain!

 

[Elric]

I'm whipping up some encounters for my next session (so my players stay out!), and while I realize we've got some guidelines from the basic DM rules (and the very cool asmor online generator others have been linking of late-- http://asmor.com/5e/monsters/#/encounter-builder ), I'm curious to see how well others have been faring.

For example, my supposedly hard BBEG from last session was basically a cakewalk for my 6 4th level PCs. (Admittedly, my group does have rather high ability scores, as I let the group keep the scores they originally had as 1st level 4e characters):

2 cultists - CR 1/8 each
2 skeletons - CR 1/4 each
1 priest - CR 2
1 modified vampire spawn - CR 4 (a lot of guesswork here in making the CR 5 into a CR 4, so perhaps this was the issue. )

By the numbers, it was a Hard encounter...but it really didn't feel that way.

The encounter guidelines have the "XP multiple for determining difficulty" for larger groups, but this multiple makes little sense when applied to a small number of strong creatures plus a few very weak creatures.

For your encounter, the 2 skeletons and 2 cultists increase the "encounter difficulty" as measured by "adjusted XP" by almost a factor of 50% over the encounter with the CR 4 vampire spawn and CR 2 priest per the invaluable site http://asmor.com/5e/monsters/#/encounter-builder. Adding an extra (CR 1/8) Cultist would have increased the difficulty of the encounter beyond "deadly" (by increasing the multiplier). But that doesn’t make a lot of sense.

The basic problem is that character and monster power doesn't scale as fast in general as the XP numbers alone would imply (e.g., a CR 5 Hill Giant worth 1,800 XP is not as deadly as 4 CR 2 Ogres worth 450 XP each). So swarms of lower CR monsters can be too strong relative to what the XP total suggests. The multiplier helps to address that design flaw.

Some monsters are legitimately much more dangerous in groups (e.g., with their own kind, like Intellect Devourers, or with any melee combatant, like Hobgoblins). Those should have been handled with special guidelines in their stat blocks, and XP should have simply scaled more slowly as CR increased (with the corresponding decrease in the XP budget encounter guidelines), thus obviating the need for a multiplier.

 

[Gobelure]

I fully agree, the system is broken for monster groups (or player parties) with significant CR (or level) difference. One CR 17 death knight and 3 CR 1/4 skeleton are not as dangerous as a CR 21 lich ! From my understanding of the system, it should only be counted as a CR18-like encounter.

The basic reason is that the encounter lethality scales as the number of monsters to the power 3/2, rendering addition between different categories impossible. Instead, you get a better math adding together the power 2/3 of the XP of all monsters and taking the power 3/2 of the sum to assess the solo-monster equivalent CR. While changes are small for equivalent-CR groups of monsters, it cures the different CR problem.

 

[Elric]

If we want to get really specific, I'm looking for feedback on throwing the same party against one CR 5 and four CR 1/2s...but that's coming up as well beyond deadly. Am I setting myself up for a TPK, or just a really tough fight? (Mods feel free to move this if it's more appropriately put in a different forum, but this seemed as good a place as any to discuss some of the general mechanics of encounter design vs. customizing a specific monster or character)

See Gobelure's fix to the encounter system. http://www.enworld.org/forum/showthread.php?367697-Encounter-difficulty-how-to-fix-it

It should approximate the regular DM guidelines when all of the monsters are the same CR, but should diverge from it (in a way that logically makes more sense) when the monsters are different CRs.

It has one CR 5 and 4 CR 1/2s as a slightly less than "deadly" fight for 6 level 4 PCs. By comparison, the previous encounter you discussed comes out at slightly more than "hard" with Gobelure's fix.