You should see if your DM will let you make your familiar, whatever you want it to be ultimately, be a spell-stitched familiar. It's a feat you take, conveniently called spell-stitched familiar, that replaces the normal benefits of a familiar with granting you 4 HD more of undead every 2 or 4 levels (AFB right now.)
And if you are going for the kobold cohort, and you're going necropolitan, see if you're able to throw levels of Master of Shrouds on there. There is a handbook around here about it, and it would fit well thematically.
Sadly, I think it's for clerics; as I said I'm AFB, so I can't check I am going to check crystalkeep.