Calliachs are;
*A race of sorcerors of incredible strength
*Enemy of the trow; driven to extinction and forced into a giant hole.
*giants (supposedly)
The Watcher is;
*2nd or 3rd most powerful sorceror in known history
*a giant (or consideribally larger than me Zerks)
*worst enemy of the Deceiver, who is the Trow's 'best friend' (if you remember from 'with friends like these')
Also, I do not really remember too many trow being in The Watcher's army. The Deceiver had them, Balor had them, Soulblighter had them, but I think all the levels where there are trow you dont fight The Watcher's army.