dark swarm

In Starcraft, a spell cast by the zerg defiler. It's sad that no one uses this amazing spell. For just 100 mana, you can make a cloud area. Any units in there are unaffected by all attacks, execpt for melee. Great for killing choke points. It doesn't work the best against zerg, since they have zerglings and ultralisks, or toss, who have good spells to attack with. However, you can still have an advantage untill they can get those unit(s) ready, which may be enough time to kill their defenses. Terrans, however, are pwned by this. They have all ranged units (besides the firebat), and no mass killing spells.

Great for siege tank whores.

Great for carrier and b/c noobs (but they weren't too hard anyway).

And once you cast swarm, you can instantly recharge your defilers by eating another unit, then casting plauge, which will just kill everything.
Yay.

I'm still amazed that blizzard almost never changed this incredibly cheap unit.
It's half the reason I play zerg.