I'm sure almost every mage wants to do this quest. 4200 mana over 30 seconds is invalueble. Here's my attempt at a detailed decription on how to obtain and complete the quest.

Navigating in Dire Maul (DM):
There are 3 sections of DM (North, East and West). Only 2 sections concern this quest, which is North and East. The questgiver is in the North, and the quest item is from the East. You will need a Crescent key to enter the North instance or a rog with max lock picking skill (Engineers, I have already tried blowing the doors up. Didn't work).

Upon entering DM from Feralas, you just need to follow the corridor till you reach a "T" path (you will notice a huge double wooden door at the T - Only graphics, cannot be opened). To the sides of the door, there are two slip-in entries. Go through both of them and it will bring you into a courtyard with ogres. Hugging the right wall will take into the east instance, left to west and straight across the courtyard is north.

Aggro range here is very small so its easy to slip past them. Dog patrols exist and they see through invis.

Step 1: Obtaining a Crescent Key https://classicdb.ch/item=35607 
This is gotten from the east instance (entering from the courtyard). Immediately entering the instance, you see a non-aggro imp on your left. Speak to him and he will start running. Fight your way in the direction he ran to (right of entering the instance). Speak to him again, and he starts running. It goes on for a couple of times.

By hugging the right wall and killing everything in the way, enter the 2nd corridor (1st one takes&nbsp you out of the instance). Go all the way straight down the corridor downstairs and hug the right wall, killing everything in your way. Go upstairs and you should see the imp there again. Talking to him will trigger him to fight. He spawns little imps over time, so focus your dps on him and deal with the non-elite adds after.

Loot the key.

Step 2: Where's the questgiver?

This can be done solo without any means of invis. Upon entering the north instance, there is a group of 3 ogres to your left and 1 wandering ogre infront of you. If you see the 3 ogres, you should also see a ramp down near them. Wait for the wanderer to move away and head down the ramp. Jump off the left side of the ramp to avoid the 2nd group of ogres, and head down another ramp.

This will bring you to another door that needs the Crescent key. Through that, all the way, takes you to the library. Questgiver is straight ahead of you, 1st bookshelf to your left.

Step 3: Getting the Hydrospawn Essence.

Eastern instance, coming from courtyard zone-in, hug the right wall, through the 2nd corridor, 1st right into a hall filled with Satyr demons and plants. Kill your way along the left wall and take the left turn below the ramp. Kill along the left wall again and this will take you to a pool with hydrospawn in it.

He spawns little water elementals as well, but they despawn when Hydrospawn dies. So focus dps on him again. Immune to cold.

Step 4: Back to quest giver.

Sorry if this seems a little long. But I didn't do any DM runs prior to this quest. I'm sure many fellow mages are alike. I hope this gives you more help than you need.

Quest giver is in the Library area of the west wing-- you will need the key from the east wing to get there. If you enter through the north and have leet skills, you can get to the Library without aggro.

The mob you have to kill is back in the east wing, the water elemental.

As of Patch 1.12 (drums of war) you can still blink through the door, 60 mage, took about 4 tries, only aggroed the one mob to die in front of first door, very simple, read the threads above

This is now trainable in patch 2.0.1. The only reason to do this quest now is to avoid the training cost. Ultimately not worth it.