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 Gracina 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 Gracina'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.
Upon completion of this quest you will gain: