Harpy/1.03

Harpies are advanced fliers.

They are interesting for their performance does not come from strength, but rather their utility. For starters, they're the only advanced flier in the game that takes up only a single army point and is of medium size, instead of the usual 2 points, large beasts, and that means that they can endure basic enemy fliers nearly as much as long as them (Harpies have fewer hits but they are not vulnerable to basic fliers). Secondly, their ability to cause poison and drain mana can leave even the victor that slain them with something to remember them by. Harpies wont be taking down many units at all, but their ability to debilitate enemy spell casters and causes ever-growing damage to even burly units through Poison, can help weaken an enemy position to the point that you can easily exploit with the rest of your army. However, due to their frailness you'll need to send in a large flock of them to be able to pull off their abilities.

Dark Elves
The Dark Elves have access to a lot of Poison dealing units, some of which are the strongest in the game. Harpies can be used to rid enemy healing-spellcasters of mana, preventing them from being able to cure them or their fellow allies of the illness, allowing it to sink in that bit longer. The Harpies Drain Mana spell can also be used on more offensive enemy spellcasters that pose a large threat to the fragile Dark Elven units. However, Harpies aren't particularly too adapt for taking on such units and require flocks of them to pull of their abilities. But once achieved, you can then swamp the enemy with Spiders, Skeletons, Assassins and Dark Archers without the fear of having them all wiped out by spells akin to Call Lightning or Ring of Fire.

Benefits from Black Ward, Summon Mana, and Harpy Hag researches.

Daemons
Harpies have a useful role, but have never really been that great at executing it. With Daemons immune to Awe, resistant to Fear, durable enough to withstand many offensive spells and with so few grounded units, Daemons aren't exactly threatened by enemy spellcasters to the same extent as other races, which further reduces the need for Harpies among the Daemon ranks. However, there is one instance that pains Daemon players, one where a bunch of Harpies could help out - debilitating High Elven spellcasters! Mystics and Ice Maidens can throw out frost type spells at your main Daemons, which certainly wont like taking the extra damage. Further more, Moonguards and Manticores have that vile Multi-Shot spell which makes short work of Imp swarms. Daemons may not have need for Harpies to overcome spellcasters from other races, but High Elven ones are painful to take on without them. It's just a shame that, as said before, Harpies don't exactly do their job well and require support just to support your other troops or need to be massed.

Benefits from Daemonic Sight, Summon Mana, and Harpy Hag researches.

Orcs
The Harpy is yet another unit that Orcs get which can inflict some kind of illness, but it wont be anything you'll be relying on. Its ability to drain mana also wont be seeing much use, either, as most of the Orc race has the durability (especially with the Training skill) to endure most damaging spells thrown at them. They can be useful against High Elves - stopping Manticores and Moonguards from casting their, otherwise deadly, Multishot spells that would normally hit Orcs, Ogres, and Goblin Shamans against their piercing weakness.

Benefits from Training, Eagle Eye, and Harpy Hag researches.

The Swarm
Most races don't respect a Harpy's traits, mostly because those races either don't need them much, or have better units for the roles that Harpies perform. The Swarm, however, can make great use of both of the Harpy's attributes. The Harpy may have the weakest poison in The Swarm army, but it can still be devastating with Melkor's Curse in play, making those foolish enough to attempt a confrontation with a Harpy soon regret it. In addition to this, the Harpy's Drain Mana spell can be put to great use. A lot of the units in The Swarm's army are physically weak and rely on numbers to "swarm" then enemy. Apart from Catapults, the main culprits for ruining The Swarm's day are spell casters, who usually carry spells that can strike multiple targets. So, before you send in your swarm of insects, lead the assault with a flock of Harpies who will deplete the enemy spell casters of their magic either through their Drain Mana spell or simply by forcing the enemy spell casters to use it. Either way, the rest of your army are free to swarm!

Benefits from Eagle Eye, Melkor's Curse, and Summon Mana researches.

Undead
Undead don't care much for Poison, due to some of their units doing large damage per strike (Slayer/Doom Knights, Liches, and Vampires), but one will always be pleased to see an enemy Hero get poisoned! However, their main use has always come from their ability to drain enemy mana and that still applies here too, but not to the extent that other races need the ability. Most offensive spell casters can be put in their place with Shadows, who are resistant to all elemental attacks. The few that aren't, such as Moonguards and Manticores, wont get past most of the Undead army, anyway.

However, Harpies can be used against more support orientated spell casters - essentially removing healing / wards and the likes, making them more susceptible to your army's assault. BUT there is one spell caster that the Harpy can be more effective against than any other unit (besides the Dragonliche), and that's the Runelord. Although it possess a fire type attack, its offensive spell, Doomstones, deals crushing damage - a type that hits nearly all Undead units for extra damage, including Shadows. This will be the one time that Harpies will prove valuable, as a Runelord without its magic is easy pickings for your Shadows. Unfortunately, Runelords can carry a lot of mana, able to cast Doomstones twice when at full. This does make things difficult for your Harpies, but if your Harpies can direct the Doomstones at them, then they have essentially done their job by saving the rest of your army from a crushing fate. If you really want to keep your Harpies alive, though, you could always send Bats in first and just fly there above them. This will probably be the only time they can be useful for fodder in the Undead race.

Benefits from Summon Mana research.

Quotes

 * Ready: "Am I not beautiful?"
 * Select: "Hee hee hee hee hee!" "Come to me." "Come hither pretty one!"
 * Order: "Yes?" "Embrace me!" "I have a surprise for you!"
 * Deathblow: "We feast tonight sisters!"