If you go around the north side of mandrake forest (out in the vamp) there is a south entrance to a walled off area that needs grappling. Once someone is through that you can nexus to them to get non-rogues in. Boss has a long repop (in the range of a few RL days) so check boss info/helpfile or gossip to see when it was killed last.
Encounter
The master of shadows is soloable for an archon and in NPK. It's possible to tag-team him but he will unform you semi regularly. Once he is almost dead you can calm and form to get all in for the boss kill.
He throws bombs, flashes, etc so web helps. Nothing else super special I think. Just calm or flee and respell heal when you need, he isn't aggro.
Don't flee west or you need to exit shadow castle and go around again to get back.
See Also