This quest was marked obsolete by Blizzard and cannot be obtained or completed.
Speak to Alchemist Pestlezugg in Gadgetzan.
It would seem that the silithid's intent, nay - reason for being, is to devour everything in their path that is not of their ecology. Their focus is singular, and it is for this reason they are a grave threat to all life on Azeroth.
What we've seen to date is but the first stages. I suspect that their strength comes from deeper in Kalimdor: Un'Goro Crater. Go to Gadgetzan in the Tanaris desert and speak to Alchemist Pestlezugg. Tell him of my theory; he will surely aid us in fighting the silithid.
Yes, I gathered that Zilzibin sent you to me. Your experience in fighting these beasties is priceless; no one to my knowledge has ever dealt with their kind before. As for Zil's idea that Un'Goro Crater is their heart... perhaps it is. Un'Goro is a mysterious and dangerous jungle to the west of here. Perhaps it is also one step closer to figuring out what the silithid are all about.