He is located in the Tigers! cave near Trelis (Gothic 3) you will find wolves in front,
and scavengers near. enter cave there will be no one there! pick up items etc, leave and
return to Montera. The brother will tell you to deal with him. Return after midnight enter
cave and he will be asleep. Get very close to him and talk, then kill him quickly!

just go and wait at the trelis gate and u will see him coming toward u

i´ve found out that if you sleep until midnight, and go to him, you´ll see him leave
him, so waiting is not needed.

heh im also at that part, but i dont think they ever told me that i needed to kill the
mage, just to find info from im

Which direction is it?

Okay As I was one that it took forever to find the black mage, I know this post is old but
was one I came across so I hope this helps. You need to go back to Anog I believe it is,
talk to him TWICE! yes thats right talked to him the 2nd time and and you get the quest to
kill him hes just where it is said in trils during the day reading, and walks out the
front and to the caves in the west. only post this as alot of people are not actually on
the "kill the black mage" quest and really trying to find him while doing "information..."
just to clear it up. Also be sure to talk to the 4-5 guys before.