The Hallow Reed is a BOP item that teaches you the Aqua Strider pet. The mob that drops this item is a lvl 90 rare spawn named Nalash Verdantis. He is located on a small island in the middle of the Lake of Stars in the Dread Wastes - and best of all, he is neutral (so you can buff up first before you kick his butt )!
First, an important note. You NEED to have interrupt abilities. He does some nasty mojo and you will die (repeatedly) if you don't interrupt the spells. Also all the abilities he has deal frost damage (in case you want to get some resistance beforehand).
Here's a list of his abilities:
Water Bolt - Interruptable. Deals a moderate amount of damage. This one you can ignore if you have enough health to soak it.
Torrent - Channeled. Interruptable. Does high amounts of damage (like 25% of your health) per tic. Use your normal interrupts here. A good thing is that he tends to give your interrupts enough time to recharge before casting this again.
Rain Dance - Channeled. Uninterruptable (by normal interrupts - you can still stop casting with stuns, etc). Launches orbs in the sky that do MASSIVE damage (like 50% of your health) when they land. Either stop him casting or get into the water. (For some reason, probably a bug, it's a lot easier to dodge in the water. Just watch out for the crocs!)
Here's his tactics (they don't seem to change) -
First Wave: WB, WB, RD
Subsequent Waves: Torrent, WB, RD (repeats this line until dead).
(Note: Just to be safe - I might have Torrent and WB reversed in the subsequent waves. Sorry if that's so. )
Manage to kick his butt and you get yourself (and they're all BOP):
1. Small Bag of Goods - it won't open (yet). Probably contains cloth and coins like the Northrend rares.
2. Piece of blue armor (don't ask me which one, my guy is stuck inside a building and unable to logon at the moment but I'll update this when that bug is fixed.
3. Hollow Reed. What you've been waiting for! Cradle it lovingly in your arms, then click on it and learn your Aqua Strider! Go forth and crush the unwilling masses with it (it's so cute that they'll never expect it!).
One last note - As for respawn time, if he's like the other rare spawn that drops a pet then people are reporting a 1hr respawn. Not that shabby! (But will probably be longer on live)