The Shadow Magic feat (Player's Guide to Faerun) gives +1 DC and +1 to caster level checks for all Illusions, Enchantments, and Necromancy (at some cost to evocations and non-darkness transmutations). A level of the Shadow Adept prestige class (also from Player's Guide to Faerun, which requires the Shadow Magic feat and any metamagic feat) will add three more feats which grant another +4 spell resistance penetration, and will make it very difficult to detect the auras of your spells or to dispel the effects of your spells.
I think it would be a fun and interesting Necromancer who took a level of Shadow Adept and kept Illusions or enchantments and got rid of evocations. Evocations are overrated, now that you have the orb spells from conjuations. Not to mention the various save-or-die/disable spells a necromancer (Ghoul Touch) and illusionist (Phantasmal Killer) or even enchanter (Tasha's Hideous Laughter) get. And lots of nasty necromancer offensive spells that have touch as a delivery method can instead be delivered at range with that spectral hand 2nd level necromancy spell.