Would just like to add in that just because he's a necromancer, it doesn't necessarily mean all of his minions are undead. Humanoid followers, aberrations and monstrosities that have been bent to his will, etc.
Thanks for the suggestions One of the things I definitely will do is use a group of lower-level ghouls (possibly iron ghouls from the toB) with a Grim Jester (also from the ToB)....that one is a giant skeletal middle finger to all heroes ...they will hate me sooo much...
Add Undead traits to the Swarm of Wasps and call them 'undead mosquitos'. The PCs meet some in the swamp. (They are an incidental side-effect of his raising other undead on purpose.)
This should creep out the PCs.