Any form of damage affects an Unseen Servant. The question is how do you target it?
This is a surprisingly difficult question to answer RAW.
First, the spell "creates an invisible, mindless, shapeless, Medium force". The spell does not say anything about damage resistances or immunities. Therefore it has no damage resistances or immunities.
Compare with wall of force:
An invisible wall of force springs into existence at a point you choose within range. […] It is immune to all damage and can't be dispelled by dispel magic. A disintegrate spell destroys the wall instantly, however.
Since unseen servant doesn't say it is immune (or resistant) to any damage types, it is not immune (or resistant) to any damage types.
The unseen servant has an Armor Class (10) and hit points (1). This strongly implies that it can be targeted by an attack and take damage, else why provide this information? So, how do we target it? PH p. 193-4:
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
- Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
As pointed out in a comment, Jeremy Crawford tweeted that the unseen servant is not a creature or object. The full tweet thread is worth quoting:
An unseen servant is "an invisible, mindless, shapeless force" (from the spell). It's not a creature. #DnD
Dr. Denica Bozhinova
Dec 19, 2017
If it is not a creature, nor an object but a "force" why the specifics of the AC and hit points.. since most effects in the game damage creatures or objects only?
Dec 19, 2017
It’s exceptional. Spells are tricksy that way.
Aug 22, 2018
Thanks for the clarification!
Per the Player's Handbook, attacks can target a creature, object, or location. Crawford says the force created by unseen servant is not a creature or object. I am pretty sure it is not a location. If we follow this logic through, the force cannot be targeted by an attack.
If this seems ridiculous to you, you're not the only one. I have to assume that @JoshPineiro1's "Thanks for the clarification!" at the end of the tweet thread is intended as sarcasm.
On the other hand, if you look at the history of the spell you find the following from AD&D:
It has no attacks, and cannot be targeted by attacks itself. It can be magically dispelled, or, if caught in an area spell, is eradicated if it takes 6 points of damage.
And this from D&D 3.5 d20 SRD:
It cannot be killed, but it dissipates if it takes 6 points of damage from area attacks. (It gets no saves against attacks.)
So at least in earlier editions, the unseen servant could not be targeted by attacks.
Therefore, the DM has a choice:
- Decide that the force is not a creature, object, or location, and therefore cannot be targeted by any attack, which is consistent with previous editions.
- Decide that since the force has an Armor Class, it can be targeted by an attack. In this case, I would not treat spells differently than physical attacks, as I do not think the fact that fire bolt says you must fire it "at a creature or object within range" means anything other than you must use the normal rules for targeting an attack. Either physical and spell attacks can both hit it, or neither physical nor spell attacks can hit it.
But I am 100% certain that if you can target it, you can damage it.