At the top of each Round, all enemy creatures within hearing of their "song" must make a WIL save (DC 10 plus the number of remaining Harpies in the encounter) or have to spend 1 Action on their next turn to Move up to their Speed toward the nearest Harpy, oblivious to danger.
Harpies are not Slowed when flying with a Grappled Medium or smaller unarmored creature.
- •Scratch & Grab.
2d6, Grappled (Escape DC11).
- •Drop on Rocks.
Fly up 4, then release a Grappled creature. If dropped over water, FALL damage is halved.
When they outnumber their prey, they will pair up to make it more difficult to escape their hold (in mechanics terms, they have to beat the Escape DC for each Harpy participating in Grappling that creature).
Focus on dropping characters into the water instead of solid ground and then attack them with just Scratch & Grab while they're in the water.
If a player shows the ability to easily escape the Grapple, have multiple Harpies Grapple that target.