You have to complete the swamp brothers quest. I missed it my first play through, talk to the brother in the castle by the Alchemist and he'll start the ball rolling. A tough fight at low level but you should easily walk through it.
After the quest is finished, the brothers offer incrementally better troops, the middle brother has a very good chance to stock Royal Snakes (If you don't know about them, get them! No retaliation and a special strike across an empty square special) The last brother Brown Hzlen or something like that has a good chance to stock Beholders, in my last game he stocked Evil Beholders
I've only seen Necromancers at the Black Tower in Demonis (Right near the end of the game)
Good luck adapting and enjoy raising the Jolly Rodger, aargh