How would you raise or lower Monster CR based on special abilities?

[mephnick]

EDIT: *Please move this to 5e forum please. My bad, not sure how to delete."

I know these abilities are generally factored in to the CR in the Monster Manual, but I was wondering about adding them on top of monster stat blocks. The Monster Manual lists creature abilities, but doesn't offer any advice on 90% of them.

How would you change a monster's CR if they suddenly were given:

Flight?
Earth Glide?
Blindsight?
Tremorsense?
Invisibility?
etc etc...

This comes up because I like to mess around with custom creatures, based on printed stat blocks. Let's say I'm creating an encounter that will involve say, the Knight statblock, but the enemy will be given the ability to use Earth Glide at will. How would you adjust for that when building an encounter, if at all? I'm tempted to just keep the CR the same and see what happens, but I'm looking for some thoughts before I go ahead.

[Firest Kathon]

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[noob]

Well the ways are often to just throw a vague estimate of how much you should increase fp but watch out if there is a world with someone able to use Earth Glide at will it might be used some minutes later to accelerate the construction of the new city of enslaved people of the player by flattening the earth and making some holes where the players wants foundations.
The players surely does not wants to kill someone with a so much convenient power if they know things like slavery(else they just murder) increasing the difficulty of the fight but the reward is great (and so the fp should not be increased for rewarding them even more)
Sorry all the beginning of the post was made with a confusion with a spell who could do some teraforming.
For the list of powers you gave the formula is to see the level of the spell giving the counter to it or the ability itself so for example flight on a monster would increase its fp to the minimum level at which you obtain it with spells unless it is not very synergistic with the monster then it might increase at most of one the fp(except if you go overboard and put ALLL then it might increase more).

[erradin]

Good question, and it is a little confusing.

I think it seriously depends on the monster and the applied ability. But, as a rule of thumb, every ability that gives the creature a form of movement the players don't ordinarily have might be a +1 or +2 depending on how deadly you feel the change makes the monster.

Earth Glide, for example, would allow a creature with fast healing to just sink underground and hang out there until it had full HP- if it was intelligent enough for that, anyway.

Any ability that grants a creature a significantly threatening ability, or else imposes a good limitation on player ability to fight back might increase the CR by 2 or 3. possibly more if it's a huge boost (ex: immunity to elemental damage, or unlimited bonus-action teleporting on a ranged opponent).

I personally find CR to be a bit vague, so I tend to wing it a little, making creatures based on how tough they should be and how likely I want hits/ saves to be. When I make a creature with a curious ability, it's usually part of a puzzle fight. (creature flies and has invisibility, but you know it won't run away. It deals lowish damage for a creature of your level. Find out how to track and hit it.)