Harpy Captivating Song

[TessarrianDM]

The area of effect is a 300' spread. Is that a 300' radius, or a 300' diameter (150' radius centered on the harpy) spread?

 

[prospero63]

The area of effect is a 300' spread. Is that a 300' radius, or a 300' diameter (150' radius centered on the harpy) spread?

I think it can spread up to a distance of 300' (so 300' radius in the context of your question). I based that on the following:

Spell Descriptions

Spread

Some effects, notably clouds and fogs, spread out from a point of origin, which must be a grid intersection. The effect can extend around corners and into areas that you can’t see. Figure distance by actual distance traveled, taking into account turns the spell effect takes. When determining distance for spread effects, count around walls, not through them. As with movement, do not trace diagonals across corners. You must designate the point of origin for such an effect, but you need not have line of effect (see below) to all portions of the effect.

If you apply the logic of a spread spell, it would seem that it should be able to go in any direction up to 300 feet away.

I could of course be absolutely and unequivocally incorrect (those words are going to haunt me, I know...)

 

[Infiniti2000]

I think it can spread up to a distance of 300' (so 300' radius in the context of your question).

That's correct. In an open, flat field, the 300ft spread would be equivalent to a 300ft radius (essentially a hemisphere). It would be less than a 300ft radius if the spread had to go around corners, such as around a building. But, 300ft is a long way.