Wanted to try a card with no mana cost that A) Doesn't have cycling, suspend, or splice and B) have an effect in the graveyard.

I figured making a zombie maker that functions in the graveyard is a start. Not sure if it's too powerful or too weak or whatnot, but I wanted to get the idea out before I forgot it.

This is reminiscent of Bridge from Below, except that this has no way for your opponent to get rid of it.
The line that declares this as black is obsolete due to the introduction of color indicators, which have been implemented on Multiverse.
Is the zombie-making ability meant to be triggered or activated? I'm assuming activated due to the "reminder text." If that's the cast, it should read ", Exile a card from your graveyard: Put a 2/2 black Zombie creature token onto the battlefield. Activate this ability only if ~ is in your graveyard and only any time you could cast a sorcery."
I'm sure that this is too powerful, though of course, I'm not a developer. The only way for your opponent to interact with it is by exiling your graveyard, and it's very easy to get this online by milling yourself. Then the cost of making the tokens is incredibly cheap. At the very least, I would require exiling creature cards.

Why is it legendary? It's not like hard-casting one, then hard-casting another one to put one of them online, is even an option.

My original plan involved putting it onto the battlefield, and the "Legendary" supertype is just a remnant of that I guess.

I missed Link's comment somehow and yeah, that wording should have been more obvious to me, especially considering it is suppose to be an activated ability. I raised the cost, but that may still be a little strong, for some of the very reasons link listed above.