You can see Help Greater Undead; here's more detail.

There are 20 new special greater undead; much as you could previously have only one mummy or ghoul at a time, you can still only have one greater undead at a time. Alternate greater undead each possess their own unique abilities but should overall be roughly as strong as a ghoul, even if they'll often be clearly better than a ghoul in some situations and clearly worse in others.

Each necromancer randomly gains the ability to create 4 of the 20 new greater undead. (This means that, finally, not every necromancer you make will have completely identical abilities.) Each special greater undead can be created in one or more specific graveyards. If a necromancer casts greater undead in one of these graveyards and is able to make the undead for that graveyard, they'll create this undead in place of a mummy or ghoul.

Let's illustrate with a fictitious example.

I go to Peter Rabbit's Graveyard and cast greater undead. If Bunnicula is one of the 4 greater undead I've "drawn", I'll make Bunnicula instead of a ghoul. If it's not, I'll make a ghoul as normal. If I cast greater undead at a graveyard that doesn't have a special undead associated with it, I will also always make a ghoul (or mummy).

The ability to get "extra" greater undead options is in our special reward toolkit as immortals; it probably will also be available as an edge at some point.