D&D 5E Determine encounter difficulty with monsters of very different CRs

[Elric]

The original encounter design guidelines are flawed and overstate encounter difficulty when you have monsters at very different CRs. I haven't looked at the update to the DM Basic Guide from November in detail. It looks much the same except that it recommends ignoring the encounter XP multiplier if the monsters have CRs that are very different. A better system would not need this step because it would already sensibly account for monsters at different CRs.

I recommend Gobelure's thread on how to modify the encounter design guidelines to fix this problem, so that you don't need an "encounter XP multiplier." http://www.enworld.org/forum/showthread.php?367697-Encounter-difficulty-how-to-fix-it The key is that in Gobelure's new tables PC and monster power scales more slowly with CR/levels, so that you don't need the "fudge factor" of a multiplier.

Here's how I described the issue in that thread:

It seems to me that PC 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 large numbers of lower CR monsters would be too strong relative to what the XP total suggests.

The encounter XP multiplier (basic DM guide, p. 57) helps to address that design flaw. However, the XP multiplier is itself flawed (as an encounter with an Adult Red Dragon and 3 Kobolds demonstrates).

My take is that some monsters are particularly 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 (CR varies based on group composition), 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.

 

[Chocolategravy]

In this specific case: yes, a drider is far more deadly with a bunch of drow along.

Maybe. A 6 character party of level 5s could quite easily kill off a fairly substantial number of CR 1/4 drow before they even get to act.

 

[FormerlyHemlock]

Maybe. A 6 character party of level 5s could quite easily kill off a fairly substantial number of CR 1/4 drow before they even get to act.

If they waste 65 HP of drow in the dark before the drow get a single action, they were going to waste the drider too, anyway. (They probably surprised the drow or something.) At least this way it gets to act while you're busy killing the drow, so you may lose a few HP to it.