Harpy hag
Dwelling
Harpy Hag is recruited in the Upgraded Harpy Loft.
Upgrading
This is the upgraded version of the Harpy.
Strategy
Their growth rate is good, they are fast, they attack without retaliation and they can fly: they are ideal for siege battles, especially with expert-level Tactics since this means they will reach their prey on the first turn.
Do cast Bless on them, though: their damage range is huge (1 to 4), so you'd rather they did 4. Especially if they have good morale.