So, my players nearly fought this one homebrew ooze made of a mercury-like substance that I gave two very dangerous traits (taking inspiration from the giant coral snake, the black pudding, and the Warlock of the Great Old One stat blocks as reference) that I did not know how to account for in CR other than increases its effective AC by one (for the 1st trait) when he is above half of his hit point maximum. This ooze is meant to be very dangerous and already has a CR of 18. How do I account for these traits in Challenge Rating calculation:

Mercurial Vapors. Every time another creature touches or is touched (with a Pseudopod Attack or Silvery Storage) by Noivilbo or hits him with an attack while within 5 feet of him, that target must succeed on a DC 18 Constitution saving throw or be poisoned until the end of that target's next turn.

Mercurial Form. If Noivilbo has an Intelligence of 1 and/or his hit points are less than half of his hit point maximum, then he reverts to his mercurial form. While in his mercurial form, Noivilbo is under the following effects:

• He gains resistance to bludgeoning and piercing damage.
• If a creature that is not immune to being poisoned fails its saving throw against Mercurial Vapors, then the target also begins to hallucinate on a failed save and is afflicted with a random short-term madness effect; the effect lasts 10 minutes.
• He cannot use his Change Shape action.

You don’t. CR is calculated by HP, AC, DPR, & Attack Bonus/Save DC:

https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingQuickMonsterStats

I understand that. However, some traits that inflict conditions can affect CR. For example, if your monster has the Stench trait, you need to increase your monsters effective AC by 1. I'm wondering if a similar thing would happen with my ooze's second trait.

Nah, resistances don’t factor in.

I'm talking about the madness inflicting effect. The fact that it could potentially inflict madness when it is touched (possibly cause someone to be paralyzed, stunned, silenced, incapacitated, fall unconscious for 10 minutes) sounds like it should be accounted for in CR.

Quote from Primed_Horyzon >>

I'm talking about the madness inflicting effect. The fact that it could potentially inflict madness when it is touched (possibly cause someone to be paralyzed, stunned, silenced, incapacitated, fall unconscious for 10 minutes) sounds like it should be accounted for in CR.

 
Sorry, I had missed that. That is waaayy too much, especially for an entire 10 minutes. (The average fight last fewer than 30 seconds.) Part of why I didn’t pick up on it is because of your wording. If I might make a suggestion:

Mercurial Vapors. A creature that touches Noivilbo, hits it with a melee attack while within 5 feet of him, or is hit by Noivilbo’s Pseudopod attack or Silvery Storage trait, that creature must succeed on a DC 18 Constitution saving throw or be poisoned until the end of it’s next turn (included in the attacks).

Transformation. If Noivilbo has an Intelligence score of 1, or has fewer than half his total Hit Points remaining, he assumes his Mercurial Form.*

—At that point, just make a separate statblock for “Noivilbo (Mercurial Form)” and add the damage resistances and “Mercury Madness” (or whatever) to that statblock and omit the Change Shape* action.—

Mercury Madness. Whenever a creature fails its saving throw against Noivilbo’s Mercurial Vapors, it also begins to experience additional mind-altering effects. The creature must roll on the short-term madness table, and suffers from the madness rolled while it is poisoned from Mercurial Vapors.

 
You had some redundant language, creatures who are immune to being poisoned automatically succeed at their Con save for example. You also had some particularly wordy explanations in a couple spots that I changed by borrowing language from other monsters like the Black Pudding you mentioned as well as adapted from others like the champion.

I also made a suggestion for how to bring the Madnesses into balance. Stuff like stunned for 10 minutes… CR 30.

I hope that helps.

*(The term “mercurial” typically indicates that something is especially prone to sudden changes or alterations. It seems oxymoronic for something to be in a specifically “mercurial form” and not be able to change shape if it otherwise could.

Could you post the monster as a whole (after making any changes you want to), or do you want to make sure it remains private?

How strong its other stats are (notably health, saves, AC, resistances, speed, DPR, etc) will severely impact how tough a monster it is.

(Btw, resistances do affect effective health, as long as they are relevant. For example, having resistance to all physical weapons that can hurt it while below half health is basically giving the creature 12% more effective health. Half the time, about half the party will be doing half the damage, so overall it will last 12% longer, or in other words, effectively 1/8 more health)

As of right now, Noivilbo is resistant to Cold and Psychic damage (without mercurial form) and is immune to Acid, Lightning, Poison, and Slashing damage. I'll boost his effective HP by 1/8 for those extra resistances and I am multiplying his effective HP by 8/5 because of the madness effect. I do not want to make his entire stat block public as of right now (since he has spells from Xanathar's Guide, it is also impossible to post him).

In addition, his CR is supposed to be under 20. Right now, his CR is 16.