Hey great question - i spent a bunch of time in here during seasons. The key dropped from the second floor, specifically 'A runnyeye foreman' as the ph for Wush (his loc is on wiki). It is definitely not a common drop, I only saw one during my few levels hanging around the second floor.
Awesome! thanks for that info. A buddy and I were killing the 3 on the first floor for speed purposes hoping we would get the drop. Sounds like we wasted our time even though the mobs we killed were still A Runnyeye Foreman.