Why is the Will o' Wisp CR2?

Question

The Will O' Wisp seems pretty challenging for a CR2 monster.

An AC of 19 is Incredibly high, especially for lower level players. In addition to its AC, it has resistance to most melee damage types and is really only susceptible to magic damage. The movement speed is also very large, and so evasion is very easy with this creature, esp. with invisibility. As if it weren't enough, the wisp has consume life, which kills downed players very quickly.

Why is this monster classified as a CR2?
What would bump it to a CR3?
Is there an Achilles heel I'm missing?

Answer 1

Calculation according to the DMG

DMG 274 has a nice table for calculating CR based on offensive and defensive stats. Running the numbers for the Will-o-Wisp:

Offensive CR

Its damage is only 9 (2d8), which is CR 1. Its attack bonus is +4, but this is not two points higher than the CR 1 attack bonus of +3, so it doesn't change its offensive CR.

It does have that instant death ability, but the DC is below the minimum in the table, and it isn't likely to be used often. The DMG advises us to use the numbers that will come up most often, anyway.

Defensive CR

The Will-o-Wisp has only 22 HP, which starts it firmly in the middle of CR 1/8. However, because of its long list of resistances and immunities, gets a 2x multiplier to that for an effective HP of 44 (DMG 277, and it's only applied once). This multiplier bumps it up to CR 1/4.

Its AC of 19 is 6 points higher than the recommended 13, which gives it a +3 to CR, making its defensive CR 3 1/4.

Total CR

The ultimate CR of the monster is the average of offensive (1) and defensive CR (3.25), which rounds to 2.

Gameplay-wise, most of the Will-o-Wisp's CR is due to its resistances, as you point out. However, because its HP is so low, it doesn't need to be hit that many times before it dies. Additionally, its damage is pretty low compared to monsters of comparable CR. You do mention mobility, but mobility doesn't actually factor into the CR (AngryGM has a great article on this, but it's tangential to this question).

Finally, its consume life ability has a very low DC, and is very situational; you'd probably have to pair it with a stronger, damage-dealing monster in order for it to become really scary. Ultimately, CR is only a basic rule of thumb, and it doesn't really take monster synergies like that into account.

If you want to bump it up to CR3, the easiest way to do that would be to increase its damage. You could increase the damage to 23 (5d8), which would put its damage in the CR3 category.

Answer 2

With plenty creatures the CR is sort of "average", as the fight can be much easier or much harder than the CR suggests depending on the circumstances, party composition, spells and items on hand.

For example mummies are CR3 in 5e, even though their fist does 2d6 + 3 + 3d6 damage (20dmg avg), and on failed save reduces maximum hitpoints until you die. In some scenarios that is a death sentence.

As for the Will-o-wisps - if you have a ranger, barbarian, fighter and monk on the party, yes, you might have an issue. If you have two people who can cast spells, they can easily do aforementioned Magic Missile or a number of other AoE. With 22hp on average they can die really easily. While it has whole array of resistances, aside of DEX it does not have amazing saving throws.

It is one of those enemies a DM must use with care - foreshadow its properties, give the party time to think about dealing with them. Running in with nonmagical weapons and expecting to just slash through, as they did with e.g. zombies or kobolds - bad idea. But a great learning experience!

Answer 3

To calculate CR, you take the average of its Offensive CR and its Defensive CR.

Calculating Defensive CR

It's base HP is 22 HP, and has 2 immunities and 8 resistances. You take its effective hitpoints here by multiplying it by 2 according to the table in DMG 277, applying the multiplier only once. Therefore, it's effective HP is 44, putting it at CR 1/4.

Now, its AC is 19, which is 6 AC above the normal AC value for a regular CR 1/4 creature. This means its defensive CR goes up 3 points, to 3.25.

Condition immunities have no effect on its CR as shown in DMG 279. Incorporeal Movement also has no effect on CR according to DMG 280. I will say Variable Illumination and Ephemeral have no effect as well, as they deal no damage or provide no protection.

Its movement speed is 50 ft (hover) -- but note that this is not a flying speed. The Will O' Wisp cannot float 50 ft upwards and make ranged attacks, so it does not qualify for the +2 effective AC as shown in DMG 279.

Our final Defensive CR is now 3.25.

Calculating Offensive CR

Its Offensive CR is simpler to calculate. Its damage per round due to its Shock action is 9 points, with a +4 to-hit bonus. This puts its CR to CR 1.

Its Invisibility ability does not affect its CR as shown in DMG 280. The only remaining ability to consider is Consume Life, but it's unlikely this affects CR calculations.

We have a final Offensive CR of 1.

Taking the average

$$\text{Final CR} = \frac{\text{Defense CR} + \text{Offense CR}}{2} = \frac{3.25 + 1}{2} = 2.125$$

We have a final CR of 2.125, which when rounded off is CR 2. So, the Will O' Wisp is strong at defense, but weak at offense.

Putting CR in perspective

This particular creature is actually not that powerful, using just raw numbers. The PCs should be able to handle them, as long as they aren't swarmed (the PCs being swarmed with anything is going to spell trouble for them due to the action economy).

• One Will O' Wisp is Deadly for a party of four level 1 PCs

• One Will O' Wisp is a Medium encounter for a party of four level 2 PCs, but two are Deadly

• One Will O' Wisp is Easy for a party of four level 3 PCs, and Hard for two Will O' Wisps

As you can see, typically, the party should outgrow the Will O' Wisp after level 2, though encountering a couple of them might be a problem.