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Suppose I want my party to fight an ochre jelly, but they're low level, so it would be too hard a fight for them. What I could give them is a smaller ochre jelly: the original jelly got split somehow, and the other half wandered off somewhere else. Maybe they'll find it later.

How would I go about calculating the difficulty of this encounter?

I do have an idea, based on the encounter mathematics, which I will post below as an answer. I welcome of course feedback and other answers, especially answers based on official or semi-official materials (supplements, published adventures, tweets, etc.)

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    – V2Blast
    Nov 22, 2019 at 4:17

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Mathematical reasoning

I am making the assumption that an encounter made up of two medium half-jellies (or four small quarter-jellies) should not be much harder than an encounter made up of one large standard jelly. In other word, a jelly is not tougher if it's already split.

This is because the jelly is expected to split as the combat goes on, and as a result the increased difficulty of fighting multiple opponents is already accounted for in the jelly's CR (which is quite a bit more than what we get by just running the numbers according to the DMG's "Creating a Monster" chapter.) It might be a bit more, because the jelly normally remains unsplit for a bit of the fight, but not much.

So since the XP value of a regular jelly is 450, the adjusted total XP of two half-jellies should be 450 as well. The encounter multiplier for two monsters is 1.5, so the actual total XP of the half-jellies is 300. So the XP value of a half-jelly is 150.

By the same reasoning, the XP value of a quarter-jelly is 50.

In terms of CR, rounding up to account for the possible increase in difficulty discussed above, that makes the half-jelly a (weak) CR 1 and the quarter-jelly a (possibly strong) CR 1/4.

Double-checking

Since the quarter-jelly cannot split any further, its CR is not affected by anything other than its immunities and resistances. So we can double-check using the DMG's math.

The quarter-jelly has 11 HP, possibly an effective 22 because of the immunities and resistances, though it does not matter, its defensive CR is 1/8 anyway. Its 8 AC is 5 points lower than expected, so it drops two steps to somewhere below CR 0.

The quarter-jelly does 12 damage on average with its attack, so its defensive CR is 1. Its attack bonus is as expected, so nothing changes.

This gives us an overall CR of somewhat less than 1/2, which confirms the above result of "strong CR 1/4".

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    \$\begingroup\$ For what it's worth, the "Reduced Threat Ochre Jelly" from Tales from the Yawning Portal has half hit points (and a to hit of +2) and is still rated at CR2. \$\endgroup\$
    – goodguy5
    Nov 21, 2019 at 20:27
  • \$\begingroup\$ @goodguy5: Note that TFTYP doesn't actually specify a changed CR, but it does say about reduced-threat monsters: "A reduced-threat monster uses a normal monster’s statistics, but it has half the normal hit point maximum and takes a −2 penalty on attack rolls, ability checks, saving throws, and saving throw DCs. [...] Some specific reduced-threat creatures also make changes to the abilities they can use. A reduced-threat creature is worth half the normal XP earned for defeating it." \$\endgroup\$
    – V2Blast
    Nov 22, 2019 at 4:25
  • \$\begingroup\$ @V2Blast Ah, I misread it. Thank you for keeping me honest. Although, 225XP is still more than the XP for a CR1 creature (200xp). \$\endgroup\$
    – goodguy5
    Nov 22, 2019 at 13:58
  • \$\begingroup\$ @goodguy5 The difference, I think, is that the reduced-threat jelly is still able to eventually split into four jellies doing four attacks per round, while the half-jelly can only split once and stop there. \$\endgroup\$
    – Aetol
    Nov 23, 2019 at 20:06

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