The easiest way to get there as the Alliance is from Stormwind. Fly (or ride) down to Booty Bay, take the boat across the Ratchet, and then ride all the way down to Razorfen. The outside of the dungeon is a bit confusing to figure out, but you'll find the entrance eventually. Hint: It's inside the big decayed pig's head. For Horde players, getting here is easy. Fly to the Crossroads or Camp Taurajo and jog down.

She's pretty deep in there, so fight through the dungeon, no need for me to tell you how, and this is where you'll find Lady Falther'ess.
While you're here, if you have Cooking, make sure you talk to Henry Stern. If your skill is high enough he'll teach you how to make Goldthorn Tea. In addition, if you have high enough Alchemy skill, he'll teach you how to make Major Troll's Blood Elixirs.

Once you open her cage, the fight begins! She fights as most banshees you come across fight/will fight. Her melee attacks do Shadow damage. She has the ability Ribbon of Souls which just does more Shadow damage. She has a Silence called Banshee Shriek, and a curse that lowers your chance to hit called Banshee Curse.

Just be careful so your healer doesn't get silenced repeatedly, and this will be an easy fight for a level appropriate group. A level 70 can, of course, tear her up without worry. Once she's dead, snag your loot.

She drops primarily healer/caster loot, but don't worry about that too much, melee players. This dungeon has a lot of nice loot including one of the first plate drops you'll encounter, so convince your party to do the whole thing. Maybe you'll get lucky and Ragglesnout will spawn with his X'caliboar. It's a fun instance, anyway. If you don't see Mordresh Fire Eye at least once in your lifetime, Sylvanas will never love you. Or uh, Varian Wrynn? Thrall? I don't know how you people swing, you freaks. You're probably into that C'Thun stuff, aren't you?

Anyway, Lady Falther'ess drops a pretty good wand for the level, Lady Falther'ess' Finger. She also drops a pretty white spell power cloak, Mantle of Lady Falther'ess.

That's all for this boss, and while there's been a bunch of these bosses one after another from the early 20s, the next one isn't until level 60. Since we're already on Kalimdor for this boss, let's take a look at Dire Maul's Revanchion next.