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The Harpy is a CR1 creature from the Monster's Manual. However, they have an interesting trait called Luring Song, which is rather unique.

Luring Song. The harpy sings a magical melody. Every humanoid and giant within 300 feet of the harpy that can hear the song must succeed on a DC 11 Wisdom saving throw or be charmed until the song ends. The harpy must take a bonus action on its subsequent turns to continue singing. It can stop singing at any time. The song ends if the harpy is incapacitated.

While charmed by the harpy, a target is incapacitated and ignores the songs of other harpies. If the charmed target is more than 5 feet away from the harpy, the target can take the Dash action on its turn to move toward the harpy by the most direct route. It doesn't avoid opportunity attacks, but before moving into damaging terrain, such as lava or a pit, and whenever it takes damage from a source other than the harpy, a target can repeat the saving throw. A creature can also repeat the saving throw at the end of each of its turns. If a creature's saving throw is successful, the effect ends on it.

A target that successfully saves is immune to this harpy's song for the next 24 hours.

Unfortunately, for the uniqueness of this trait, it's not listed in the DMG (pg 280-281) how this feature affects its challenge rating. This is unfortunate as this prevents some more meaningful, calculated adaptations of this same skill onto other custom-built monsters.

How would Luring Song affect the Challenge Rating of a given monster, and/or in isolation, in the way displayed in the Monster Features table in DMG pg. 280-281?

  • Can you find a similar creature without luring song, and compare the CRs? – BlueMoon93 Mar 03 '17 at 13:58
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    Have you tried re-calculating the Harpy's CR and disregarding luring song? – daze413 Mar 03 '17 at 14:16
  • @daze413 Yes I have. Without Luring Song, it has a defensive CR 0 and an offensive CR 0 –  Mar 03 '17 at 14:20
  • Keep in mind that the listed CR might not be using the exact rules from the DMG. See this question – Sdjz Mar 03 '17 at 14:50
  • @Sdjz While this is true, it doesn't seem particularly applicable to my question. The ogres have no defining trait except for the high swing between offensive CR and defensive CR (2 and 1/4), so CR bump is understandable. The Harpy doesn't have this swing. Luring Song is the only defining trait of the Harpy, and given it can bring a CR 0 creature to CR 1, it must have some powerful effect that can maybe be quantified. That's what I'm interested in knowing. –  Mar 03 '17 at 14:56
  • By my calculations, the Harpy's OCR is 1 and its DCR is 0. – daze413 Mar 03 '17 at 15:13
  • @daze413 Its damage adds up to 9 per round due to multiattack, which puts it at OCR 1. However, that's assuming it never uses Luring Song. If that's taken into consideration, then it deals 0 damage on turn 1 to activate LS. Since the DPS becomes variable, then we average the first 3 rounds of damage. 0 damage on turn 1, 9 damage on turns 2 and 3 each. The net DPS is 6, putting it in the realm of CR 1/2. However, the save DC for that is 13, but the DC for LS is 11. Two points lower means knock a point off CR, so I calculate OCR of 0. –  Mar 03 '17 at 15:17
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    @markovchain you dont factor save DCs in OCR, only attack bonuses. And, calculating OCR, you assume the creature always uses its most damaging attack option. In this case, a harpy would never use luring song when youre calculating OCR. – daze413 Mar 03 '17 at 15:38
  • @daze413 Yes, you may factor DC into the equation. DMG pg 275: If the monster relies more on effects with saving throws than on attacks, use the monster's save DC instead of its attack bonus. And a creature who fails this save is incapacitated, triggers opportunity attacks, and so becomes unable to do anything. I would think Luring Song does give the Harpy more effective damage and defense. Note, it can affect more than 1 creature at the same time -- potentially multiple PCs brought out of battle with a failed save. –  Mar 03 '17 at 15:58
  • @markovchain Not only that, but it can be used as a save-or-die effect. Granted, they'll need to fail two saves to fall victim, but it could be particularly nasty against low WIS characters. – LegendaryDude Mar 03 '17 at 16:05
  • @LegendaryDude Agreed. And this is just one Harpy. –  Mar 03 '17 at 16:10

2 Answers2

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No adjustment is necessary.

I asked Chris Perkins, the lead designer of the Monster Manual, this question for you.

Me: If I add Luring Song (see: Harpy) to a custom monster, how much CR adjustment do you recommend? It's not in DMG 280-281.

Perkins: I wouldn't make any adjustment to the creature's challenge rating.

I think his word should be considered reasonably authoritative on a question like this, since he did oversee the entire Monster Manual design. The response is in this tweet.

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Bloodcinder
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From general observation, a creature's Action that takes up its whole action and does no damage generally do not increase CR. Another ability, "Leadership", comes to mind, and it's listed as having no effect on CR.

Likely, the Harpy's final challenge rating of 1, which is pretty high, is a result of playtesting. The DMG even says as much:

Creating a monster isn't just a number-crunching exercise. The guidelines in this chapter can help you create monsters, but the only way to know whether a monster is fun is to playtest it. After seeing your monster in action, you might want to adjust the challenge rating up or down based on your experiences.

Based on these, I'd give Luring Song no effect on Challenge Rating and give your homebrew monster a conservatively low CR to srart with, it's easier to adjust a creature with CR too low.

daze413
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  • I'd like to point out, just as an observation, that Luring Song is effectively a save-or-die effect when used as intended. – LegendaryDude Mar 03 '17 at 15:55
  • @LegendaryDude save once or die - it gives at least one save per round. – Dale M Mar 03 '17 at 21:39
  • @legendarydude yeah but it's too easy to break out of. Taking damage, saving at the end of the turn. Even the example I used, "leadership" as a direct buff to all allies of a hobgoblin captain, doesn't improve CR. Neither will this ability. – daze413 Mar 04 '17 at 00:03
  • That it can be broken out of easily is measured by the DC, not the trait itself. The trait itself imparts some substantial increase in CR. Imagine a dragon with Luring Song. Mass Incapacitated (DC 20 save) plus Breath Weapon. Should the CR not increase? –  Mar 04 '17 at 04:11
  • I wouldn't increase it. Imagining this hypothetical dragon, I'm thinking 1st round Luring Song (gets a save, and as high level characters have so much ways to pump wisdom saves or be immune to charm, DC 20 isn't that devastating, IMO) and then the Dragon uses a breath weapon is really just a waste of Action as a Dragon can just use its breath right away, sure you might get an additional PC in the AOE, but it's situational. Remember, Incapacitated doesn't penalize any saves, so the creature still can make a Dex Save as normal. And, when they get hit by the breath, they snap out of it. – daze413 Mar 04 '17 at 04:41
  • To reiterate, though, when I say I wouldn't increase it. This is before playtesting. If playtests come out that it is a bit tougher, then an increase is in order. Again, playtesting, here, is the final step. – daze413 Mar 04 '17 at 04:45
  • The fact that, once you save against the harpy's song, you are immune to it for a day means this is not strictly a save or die feature. However, add one more harpy and the risk rises considerably, and it continues to rise with each extra harpy. The question is, does this make the difficulty of an encounter with extra harpies grow faster than it does for other monsters - I think it does, but can't put my finger on why! To an extent it probably depends on how the Luring songs of several harpies affect the PCs at the beginning of the encounter. Hmmm – Clearly Toughpick Mar 04 '17 at 08:33
  • @daze413 They don't snap out of it if they take damage as a result of being attacked by the one who charmed them. And playtesting is fine and all. But your answer is essentially "I don't know, playtest it to find out" which is not necessarily what I'm after. –  Mar 04 '17 at 08:54
  • @ClearlyToughpick If there were 2 harpies and they went at the top of initiative, everyone would make 2 Wis saves. If they failed the first, they ignore the second harpy's song until they snap out of it. As soon as they snap out of the 1st harpy's song however, they must make another Wis save because the 2nd harpy's song is still ongoing. –  Mar 04 '17 at 09:09
  • I'm saying there's no effect, and that playtesting is the final step. Not "I don't know, and you should playtest" that's because monster building isn't number crunching. There's a huge difference, IMO. – daze413 Mar 04 '17 at 09:13
  • @markovchain I'm thinking you have huge misconceptions about how strong this ability really is. It really is not that strong. – daze413 Mar 04 '17 at 09:14
  • @markovchain But how do you choose which harpy each character saves against? Do you assign them randomly? Pick one harpy to go first against all characters, moving on to the next when a character succeeds on their saving throw (which is what you seem to have done)? Share the characters out evenly between the harpies? The chosen method of allocation will have a big impact on the challenge of the encounter. – Clearly Toughpick Mar 04 '17 at 09:48
  • @daze413 I think he has it right, and it could be even more powerful if you evenly distribute the PCs amongst the harpies, rather than choosing one to go first. Out of interest, how do you differ to @markovchain? – Clearly Toughpick Mar 04 '17 at 09:53
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    @ClearlyToughpick I'd go by initiative order, because technically not all harpies will be able to sing at the same time. It's an action to sing, so they have to spend their full turn doing just that, which is determined by initiative. –  Mar 04 '17 at 17:07
  • @daze413 I'm quite open to being shown it's not powerful. It's just hard to show that if I have to playtest it to get to that conclusion... However, I also have the impression that if you encounter a harpy in already dangerous terrain (lava, high cliffs, deep bodies of water, maybe even around a sphere of annihilation), and you encounter more than one, you may very well die from non-direct damage from the harpy. –  Mar 04 '17 at 17:11
  • @markovchain I'd like to playtest this as well. Care to make a chatroom for it? :D – daze413 Mar 05 '17 at 00:45