By Alan McCoy from Dungeons & Dragons: Fundamentals
How to build challenging encounters against a Shadow
Note: This has been prepared utilizing only the 5E Core Rules. The Shadow can be found in the Monster Manual page 269.
Often discounted as a lesser undead spirit, the shadow is probably one of the most fearsome monsters known. While appearing as little more than darker than normal splotches in areas that are already full of darkness, the Shadow is virtually invisible unless a strong light source reveals them.
There is strong evidence that mankind’s ancient fear of the dark is more than a fear of a the simple unknown or loss of light, it in truth a group fear of what lurks on the edges of the darkness: the shadow. From time beyond memory, these monsters have been on the edge of civilization, feeding themselves with the vitality and strength of living beings, consuming the living flesh, leaving nothing behind. Magical investigation has yet to determine which actual part of the individual is consumed, the flesh, the vitality or the spirit, there is evidence that supports that any or all of these could be consumed.
A victim, once completely drained of vitality and energy, will rise as shadows themselves, often in mere hours, to join the other shadows in their eternal hunt.
Attempts to resurrect or otherwise restore an individual who has risen as a shadow are usually not successful, although victims who have not yet risen have been restored.
Step 1) Let’s Review what we know about the Shadow.
While they are physically weak, they are very dexterous and tough, they operate on an intellectually level that is only slightly greater than instinctual but have a strong will to survive and enough mental energy to maintain coherence and concentration on a goal.
Shadows are naturally stealthy, especially in dim light or darkness. If caught in bright light, they will attempt to merge with a naturally cast shadow to maintain stealth.
Shadows have a damage vulnerability to RADIANT energy.
Shadows are resistant to damage from ACID, COLD, FIRE, LIGHTNING, THUNDER, AND WEAPON ATTACKS DELIVERED BY NON-MAGICAL WEAPONRY.
Shadows are completely immune to Necrotic and Poison damage.
Shadows are Conditionally immune to Exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone or the restrained conditions.
A Shadow has Darkvision (60-foot) and a passive perception of 10.
A Shadow cannot speak but does understand the languages it spoke in life.
A Shadow is Amorphous, which means that they can move through any space at least one inch wide without squeezing or even slowing.
A shadow can HIDE as a bonus action if in dim light or darkness.
While in bright light, especially sunlight, the Shadow suffers disadvantage on all attack rolls, ability checks and saving throws.
In addition to their tirelessness, they have a movement speed of 40-feet per round. (NOT RAW – I grant them immunity to difficult terrain and the ability to glide over water or other obstructions that would slow a corporeal being.)
Step 2) Determine the probable Strategy.
Quite simply, to a Shadow the living beings are food. Relentless in the hunt, it does not tire, rest or give up once it has chosen a target. It pursues until it successfully feeds or is destroyed. If the shadow finds that it faces significant opposition, it will move away long enough to find others who are less prepared, drain and convert them to Shadows and allies, then continue the hunt for its primary prey.
The Shadow will avoid direct sunlight, trailing its prey from shadow to shadow until twilight and nightfall gives it the ability to attack full force once again.
Step 3) Determine Tactics
While the shadow does have a physical attack, that does significant damage, its major ability is to drain the strength from its victim. (1d4 points per successful attack)
Note that the Shadow does not need to be in a standing fight to attack, the presence of any body part into the shadow will provide it with an opportunity.
As stated above, the shadow is not averse to creating other shadows to assist it in bringing down a significant or otherwise difficult kill.
If a target is reduced to ZERO strength, it dies and will rise as a shadow in 1-4 hours.
A shadow can affect the physical world, and will when possible, limit the available sources of light by extinguishing torches, turning out lanterns, and either extinguishing or otherwise dampening campfires. Permanent or magical light sources will be removed, covered or disposed of whenever possible.
Step 4) DM Tips and Observations
As an added feature, I enjoy giving shadows the cantrip thaumaturgy to allow them a magical excuse to affect light sources. I find it makes them a scarier monster.
As mentioned above, because Shadows are effectively insubstantial, I make them immune to difficult terrain and give them the ability to glide across water or other terrain features that would slow a corporeal creature.
If a skeleton or other corpse is available, I will allow the shadow to speak using the voice and inflections of the deceased. The conversation is never of much use, mostly a litany of how the shadows will enjoy devouring the life force of the potential victims.
If you want to view an example of Shadows in action, I suggest an episode of Doctor Who: Silence in the Library, featuring the first appearance of River Song and the Vashna Nerada as the Shadow Creatures that have eaten their way through almost the entirety of those visiting the library during their attack.