Does Fury of the Small apply the extra damage to each hit/attack, or only once per turn?
The goblin playable race (Volo's Guide to Monsters, p. 119) has the "Fury of the Small" trait:
Fury of the Small. When you damage a creature with an attack or a spell and the creature's size is larger than yours, you can cause the
attack or spell to deal extra damage to the creature. The extra damage
equals your level. Once you use this trait, you can't use it again
until you finish a short or long rest.
Say there's a Goblin Fighter with Extra Attack; does the extra damage apply for every single attack, as each hit is dealing damage and they are all part of a single Attack action? Or is it only applied to one of the attacks during that action?
Likewise, a Goblin Warlock with max level eldritch blast; does the extra damage apply to each beam since the blasts are technically considered one spell cast? Or is it because you are making multiple spell attack rolls that they are each considered separate, and the damage only applies once?
Also, when casting an AOE spell, do all of the enemies hit by the attack suffer the extra damage added on by this trait, or must you simply pick one? Or, because it says when you damage a (meaning a single creature), you can't activate it when you hit "multiple" targets?
Best Answer
It activates only once
The trait reads (emphasis mine):
Notice how "attack" is written in lowercase. This indicates a single attack. Not an Attack Action. When you have extra attacks, you attack multiple times with one Attack Action. But when you activate Fury of the Small for example on your first attack of your multiple-attack Attack-Action:
This means you can only use this effect on one of your attacks. The same applies to spells.
Except when you use AoE spells
In the comments Erik raised a very interesting point for this discussion:
The difference is that an area spell is still one single spell and that you don't make a spell attack. The trait does not limit the effect to spell attacks, but instead to spells that cause damage. Instead the targets make saving throws. That means that if you use this trait with an area spell like Fireball you should be able to add the extra damage to every single target that takes damage from your spell. Note that some AoE spells also have a clause that mentions a target taking damage even if they make their saving throws. These targets would still be valid targets for the additional damage.
If you instead use the Eldritch Blast you have the same situation as I already described in the case of multiple-attack Attack-Actions: you attack multiple times with a spell attack. This means you can only add the damage to one of these attacks.
Personal Addendum
I feel like the described AoE damage increase is not really the intended use of the trait, but according to the Rules as Written you would be able to add the extra damage to any target that takes damage because of your AoE spell. As a DM I might rule that the trait should read:
This would disallow AoE spells and other spells that use saving throws. It would only apply to real attacks against single targets and could only be used once.
Another way of changing the trait might be:
This would make it clear that the damage has to be focused on a single creature. In general this would disallow AoE spells, but then again it may be the case that it just so happens that only one enemy was caught in your Fireball, which would allow this again. While the second version might be interesting for some people as a way of changing the trait to deal with the problem of applying the damage to multiple enemies, I would recommend to use the first "fix" to simply disallow AoE spells.