The Monster Manual contains a template whereby any true dragon may be turned into a shadow dragon. A shadow dragon differs from a true dragon in the following ways:
- It has resistance to necrotic damage always, and resistance to all other forms of damage except force, psychic and radiant, when in dim light or darkness.
- It has double proficiency in Stealth.
- It can Hide as a bonus action when in dim light or darkness.
- It has sunlight sensitivity (disadvantage on attack rolls and Perception checks involving sight when in sunlight).
- The damage type of both its bite and its breath weapon is necrotic (regardless of type of dragon).
- A shadow rises from the corpse of any humanoid killed by its breath weapon.
Otherwise, the shadow dragon has the same stats as the true dragon. How do you evaluate these differences to come up with a new Challenge Rating? Right now I am specifically looking for the CR of a Black Shadow Dragon Wyrmling, but the best answer will explain how to determine the CR of all types of shadow dragon.
One clue: The Monster Manual provides one example stat block for a shadow dragon: a Young Red. The CR of a Young Red Shadow Dragon is 13, while the CR of a Young Red Dragon is 10. Is merely adding 3 to the CR a good rule of thumb?