a reborn necromancer lawful neutral awaken in a crematorium to find out he was dead for days. (he studies magic to try and reverse what happened to him)

so what you think about a fun familiar for him???   

The two most obvious to me are: 

A cranium rat that telepathically tells him it is responsible for animating him and treats him as if he's the familiar and it's the master.

Or one of his hands is a crawling claw. But I'm a big fan of Evil Dead 2 and I thought that severed hand was hilarious. 

OMG i love that movie too i have them all

If you go that route, he'll need a Staff of Thunder and Lightning.

Something like a psudeodragon is good. A young Fearie Dragon could be kind of busted as a familiar, but cool.

You can change the familiar every time you cast it (bat, rat, raven, owl, spider, lizard, hawk, weasel, etc). You could say everything you summon is jet black if you want a dark theme.

Or, If you happen to have a strixhaven background...you can get a special familiar if you take the Strixhaven Mascot feat, and if you choose witherbloom college (common for necromancers), you get the Pest Mascot as a familiar. Plus, you'd get some healing spells in that college.