Shadows are a CR 1/2 monster, You can find shadows on page 269 of the 5th Ed Monster Manual, and while this may make them appear to be weak and ineffective once you really delve into the shadow’ s stats and abilities you find that the opposite is true. They are actually extremely potent monsters if used effectively.
Let’s have a look under the hood, shall we? The first thing that we notice about a shadow is the stealth skill. They get a +4 to stealth, now this becomes a +6 in dim light or darkness making them virtually impossible to see in the dark. This makes the shadow an optimal ambush predator. They have dark vision up to 60 feet and a passive perception of 10. They have a host of resistances among them being acid, cold, fire, lightning, thunder, bludgeoning, piercing, and slashing from non-magical attacks. They are immune to necrotic damage and poison but have a vulnerability to radiant magic. Just these resistances and immunities alone make them extremely challenging for a low-level party and I daresay with the proper DM at the helm they could be quite deadly even for a high-level party.
These monsters are amorphous, they can move through tiny spaces without squeezing and while in dim light or darkness they can take the hide action as a bonus action. They do have sunlight weakness which grants them disadvantage on attack rolls, ability checks, and saving throws but what DM in their right mind is going to use shadows in broad daylight?
Here’s where the shadow really become scary. Their strength drain ability deals 2 D6 +2 necrotic damage and the target's strength score is reduced by 1d4. They get a +4 to hit with this attack. Once the victim's strength hits zero, they die, automatically. Now, if this target happens to be a non-evil humanoid, and they die from the strength drain attack, a new shadow rises from the corpse 1d4 hours later. This, I believe, is what makes them a particularly frightening monster especially with such a low CR.
Image from D&D beyond
The Monster Manual says that a shadow is an undead creature that resembles the dark exaggeration of humanoid shadows. It looks like a shadow cast by you or I, but there is something different about it something sinister, something unnatural.
They have no features, there is no discernible mouth, nose or ears just a vague outline of their host body.
These monsters live by feeding on the life energy of living things. They have no preference as to what living thing it is that they drain of energy but they are drawn by individuals who are untainted by wickedness and evil. This is their preferred prey, perhaps the taste of goodness, righteousness, and virtue is sweeter than that of evil.
Shadows can be very threatening. Consider this, a shadow gets loose in a city, just one shadow. Over the course of one evening, how much damage to the population of the city do you think the shadow can do? How many individuals, common people, do you think the shadow could transform into other shadows over the course of this one evening? A staggering number I'd wager.
Put a group of weary adventurers in a tavern for the evening in a city where one single shadow is running loose and you have the makings for a wonderful scenario. Escape the city where the population is quite rapidly being transformed into shadows. Survive the bedlam that is no doubt going to ensue. The population of the city has the potential to be just as dangerous as the shadows that are transforming these individuals every night.
There’s a lot of ways you can use a shadow in your campaign, and this is just one.
See you next time in the Dojo