Rise of the Silithid quest chain

... wherein you investigate the infestations of insect life arising in Tanaris, and find a threat to all Azeroth.

This series of quests is available to both Horde and Alliance; the differences being to whom you report the news. Also note that there is a level gap between Rise of the Silithid and March of the Silithid which may make the second half of the chain temporarily unavailable.

Part 1: Rise of the Silithid
Senior Surveyor Fizzledowser is looking for assistants to go looking for water sources for Gadgetzan. It seems that the nomads have been seizing all the wells, and he is looking to find more water sources. The pay sounds good, the work easy, so you volunteer.
 * Gadgetzan Water Survey

Fizzledowser hands you a dowsing widget, and asks you to head to a pool near the troll-held Sandsorrow Watch. You find the pool. Carefully avoiding the trolls, you use the dowser. But your sampling is interrupted by a couple of pony-sized insects burrowing out of the ground at your disturbance.

You return with the dowsing results. Fizzledowser apologizes for not mentioning the possibility of the insects, as he had thought them mere inventions of the nomads to cover their water thefts. Now he knows better.

And knowing your foe is the first step to being able to control or defeat a threat. Fizzledowser asks you to go to the Noxious Lair and bring back enough parts for Alchemist Pestlezugg to investigate.
 * Noxious Lair Investigation

While you can get plenty of parts (carefully - those are bad bugs), getting useful parts take a bit longer. Still, you manage it. You return with the parts to the alchemist, who tells you that Fizzledowser has had some emergency come up that he'd like your help with. The analysis will take some time anyway; you have time.

Fizzledowser, it turns out, had sent his top assistant, one Junior Surveyor Scrimshank, out to Gaping Chasm, and he's gone missing. Fizzledowser would like you to go out and find out what happened to him. Rescue him if he's still alive, or at least retrieve his equipment.
 * The Scrimshank Redemption

You don't find sign of him anywhere on the surface, so you head into the nest. Quite some distance in, you find what is left of his survey equipment, where the bugs probably stored it.

You return the survey equipment to Fizzledowser. He asks you to check in with Alchemist Pestlezugg to see if he's got results from those parts you gave him.
 * Insect Part Analysis

He does indeed, it turns out. Those results seem to confirm that the bugs are being controlled by some outside force, and a pretty malign one at that.

Pestlezugg insists that Fizzledowser needs to see this immediately. Not only that, but he feels you should show the report to anyone you can get to sit down and read it. The threat, he says, is not only to Tanis and Gadgetzan, but to all of Southern Kalimdor, possibly to the whole of Azeroth.
 * Insect Part Analysis (2)

Sharing (for free) does not come naturally to Senior Surveyor Fizzledowser, but Pestlezugg is a long time associate, and he trusts the alchemist's judgement.

The Senior Surveyor urges you to take the report to a scholar who has studied this sort of thing.
 * Rise of the Silithid (Horde) /Rise of the Silithid (Alliance)


 * Alliance members are directed to Gracina Spiritmight, a night elf scholar living at the Temple of the Moon, in Darnassus.
 * Horde members are directed to Zilzibin Drumlore, a troll scholar living in the Drag in Orgrimmar.

The scholar you are sent to agrees with Fizzledowser, that this threat is real and serious, to both Alliance AND Horde.

They even have a name for it: Silithid.

Part 2: March of the Silithid
Your scholar contact asks you to return to Alchemist Pestlezugg in Gadgetzan and relay to him that he believes the direction is coming from Un'Goro Crater.
 * March of the Silithid

Pestlezugg is very glad for your experience fighting the Silithid. He's a little less sure that Un'goro is the ultimate source, but it's a start.

The alchemist asks you to venture into Un'Goro and retrieve an intact scent gland from one of the Silithid, as well as several samples of the native Un'Goro soil. He believes that if the Queen can be drawn out of hiding and slain, the rest of the hive will be disrupted.
 * Bungle in the Jungle

The soil you find easily enough. Clods of it stick to your shoes all the time. The scent gland is a bit more difficult, as the process of killing the bug often renders the gland unusable. Still, you manage, and return with the parts.

Pestlezugg brews up the lure and advises you on how to use it. Somewhere in the hive, he says, is a crystal that they use for communication. Putting the lure near the crystal should draw the queen - and her bodyguards - out where you can get to her.
 * Pawn Captures Queen

You venture into The Slithering Scar in Un'Goro Crater, and find a hive much smaller than either of the ones you found earlier. You find the crystal Pestlezugg mentioned fairly easily and lay out the lure.

As hoped for, the queen comes out. And as predicted, a number of bodyguards accompany her. With some assistance, you finally manage to dispatch them. As requested, you slice out the queen's brain, and bring it back to Pestlezugg for analysis.

Alchemist Pestlezugg puts together another report, on the events in Un'Goro, and asks you to return to your scholar with it, and asks you to pass on the request for more aid.
 * Calm Before the Storm (Horde) /Calm Before the Storm (Alliance)

... and disconcertingly, the scholars on both sides are similarly frustrated. The leadership of both the Horde and the Alliance are so fixated, each upon the other, that they are almost certain to ignore this threat.

Your scholar contact thanks you, but warns that you are certain to be called upon again some time soon, to assist against the Silithid further. The results from your foray show that instead of being directed from Un'Goro, there exists a far stronger presence elsewhere.
 * Calm Before the Storm (Horde 2) /Calm Before the Storm (Alliance 2)

In the mean time, she presents you with a note to take to an agent at the bank. You present the note, and are given an item from the scholar's vault as a token of gratitude.

Rewards
Over the course of the chain, you earn amazing amounts of faction: You also earn
 * +850 Alliance/Horde
 * +1160 Gadgetzan
 * +250? Steamwheedle Cartel
 * +10 Darnassus/Orgrimmar
 * 38300 XP in quest completion awards