Viscous Fallout has coalesced from that mass, and formed its own consciousness, and its own agenda.
Viscous Fallout is quite easy to farm for a high-level rogue or druid, as you can reach him by simply dropping off the cliff as you enter the main Gnomeregan entrance. After killing him, you can stealth back to the front and reset the instance. Your lower-level party members don't even need to drop down, as they can roll for the item while staying on the top level.
Group leaders or groups unfamiliar with Gnomeregan may not realize this mini-boss exists, as it hardly differs in appearance from the rest of the mobs in the area. It may also be useful to have a /target macro readily available. Doing so may allow the group to track and slay the mini-boss without having to fight through groups of enemies to find him.