You have mentioned the tip of the iceberg for what can be done with the undead. This is a good topic and I'll just respond by listing undead unit types. In random order and I may have missed a few, but here's my personal opinion of undead types:
1) Ghosts: Upgraded types of ghosts include Ghasts, Wraiths, Sceptres and even the dreaded Poltergeists. No need for a master, but may be controlled by a master.
2) Skelletons: No real upgrades for these units and they cannot gain experience. Needs a master.
3) Zombies: There are different types, but most are mindless which follow a master.
4) Mummies: A ritual is performed which preserves and enhances the body. A mummy is very strong in one or more statistics... usually strength. Although the brain is gone, the soul controls the body. No need for a master, but may be controlled by a master.
5) Vampires: A powerful vampire can create other vampires, but they will be weaker than its creator. For example if the original Dracula creates a vampire then it would be a very very powerful vampire. This process may take several days and is not always successful where the target victim dies. Usually they follow another more powerful vampire.
6) Necromancers: Any being which has Necromancy skills and the necromancer usually starts as a living being with curiousities in death.
7) Liches: A ritual is performed by necromancers to become a Lich because of fear of approaching death or old age. As a lich the necromancer is now undead and also has enhanced necromancy skills. This ritual is usually only 33% successful and costly with resources and time. The Lich can later perform another ritual to become a demi-lich which provides a type of immortality as well as even stronger necromancy skills. This second ritual is usually only 20% successful and is very costly with resources and time.
8) Frankensteins: This is a type of monster zombie which takes the parts from multiple dead beings and/or creatures to create a very powerful undead which is sometimes mindless. Creating this creature is a very difficult task.
9) Reapers: One of the most feared of all the undead and usually does not have any master. This powerful undead has pure chaotic actions, but sometimes focused on a specific mission. Rumor has some demi-liches are able to summon them for specific missions.
10) Dracoliches: They are created from a combination of very powerful necromancy magic and the corpse of a dead dragon. Even if someone is lucky enough to find a dead dragon and then lucky enough to successfully perform the ritual in creating the Dracolich it's possible this monster will remain chaotic. The Dracolich has some of the previous Dragon abilities and some Lich abilities.