What would a Shadow Pseudodragon's CR increase to?

Question

I want to create a shadow version of the Pseudodragon for a Pact of the Chain warlock's familiar, and wanted to know if it would be unbalanced. It would gain the gain all the features a dragon gains when it turns into a shadow dragon (see the Shadow Dragon Template on MM p. 84-85), except the new Bite and Shadow Breath actions:

Damage Resistances. The dragon has resistance to necrotic damage.

Skill Proficiency: Stealth. The dragon's proficiency bonus is doubled for its Dexterity (Stealth) checks.

Living Shadow. While in dim light or darkness, the dragon has resistance to damage that isn't force, psychic, or radiant.

Shadow Stealth. While in dim light or darkness, the dragon can take the Hide action as a bonus action.

Sunlight Sensitivity. While in sunlight, the dragon has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Compared to the Pseudodragon (originally CR 1/4), the Imp (CR 1) gets at-will Invisibility, shape-changing, immunity to fire and poison damage, and non-environmental conditions for resistance to cold, bludgeoning, piercing, and slashing from nonmagical attacks that aren't silvered.

So would the additional features push the Pseudodragon up to CR 1 to match the Imp, or would it be less/more than that? If it is still less than the Imp, would adding 1d4 or 1d6 necrotic damage to the bite attack help to push it to or past CR 1?

Answer 1

It is close to 1/2

Lets assume it can stay in dim light. Sunlight Sensitivity is listed as a feature that does not change CR in the DMG anyway (p.281).

Defense CR = 1/2

Damage Resistance and Living Shadow can be counted to give it resistance to pretty much everything. The types not covered by this are usually affected by Magic Resistance. Doubling its HP for CR calculation purposes should cover these neatly. This is not enough to chance its defensive CR.

The other big defense bonus comes from Shadow Stealth. This is actually covered by the DMG on page 281. This trait gives 4 effective AC to the creature, which is 2 bumps on the defense CR scale.

Offensive CR = 1/4

Not much has changed here. The monster can try to hide every turn (like Nimble Escape), but has no ranged options or the ability to disengage better. Even if you count the poison, this is at most 1 bump, up to 1/2, but even that is shaky.

Overall

Which way you round is up to you, and a CR 1/2 is a good guess, which is all you can hope from a CR. Many abilities rely heavily on circumstances (light), so the actual challenge will change with that too. Do not depend on a CR calculation when building encounters if you can.