Do PCs facing a group of harpies using Luring song at the same time make a wisdom saving throw for each individual source of the melody or is it a single roll, considering it's the same ability?
Here are the relevant parts of the ability's description (MM, p. 181, emphasis mine) :
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. [...] While charmed by the harpy, a target is incapacitated and ignores the songs of other harpies. [...] 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.
My reading is that once PCs have made a successful check against one harpy, they are immune to this harpy's song for 24 hours, but must still make saves against every other singer until they fail on one roll or succeed on every last one of them. Is this right? It seems powerful to me.