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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?

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  • \$\begingroup\$ Can you find a similar creature without luring song, and compare the CRs? \$\endgroup\$ – BlueMoon93 Mar 3 '17 at 13:58
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    \$\begingroup\$ Have you tried re-calculating the Harpy's CR and disregarding luring song? \$\endgroup\$ – daze413 Mar 3 '17 at 14:16
  • \$\begingroup\$ @daze413 Yes I have. Without Luring Song, it has a defensive CR 0 and an offensive CR 0 \$\endgroup\$ – user27327 Mar 3 '17 at 14:20
  • \$\begingroup\$ Keep in mind that the listed CR might not be using the exact rules from the DMG. See this question \$\endgroup\$ – Sdjz Mar 3 '17 at 14:50
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    \$\begingroup\$ @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. \$\endgroup\$ – daze413 Mar 3 '17 at 15:38
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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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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.

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  • \$\begingroup\$ I'd like to point out, just as an observation, that Luring Song is effectively a save-or-die effect when used as intended. \$\endgroup\$ – LegendaryDude Mar 3 '17 at 15:55
  • \$\begingroup\$ @LegendaryDude save once or die - it gives at least one save per round. \$\endgroup\$ – Dale M Mar 3 '17 at 21:39
  • \$\begingroup\$ @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. \$\endgroup\$ – daze413 Mar 4 '17 at 0:03
  • \$\begingroup\$ 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? \$\endgroup\$ – user27327 Mar 4 '17 at 4:11
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    \$\begingroup\$ @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. \$\endgroup\$ – user27327 Mar 4 '17 at 17:07

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