Game Mechanics: The Undead

It wouldn't be D&D without the undead. Skeletons and zombies are among the standard low-level threats to chuck at a beginning party because they're dangerous but not overpowering. Plus, let us face facts directly here, skeletons with swords are cool. Defeating a swarm of zombies when you're outnumbered is cool. Plus, in 1899 world there are no orcs, elves or dwarves so I need some kind of antagonist to throw at the party. (There are goblins and kobolds, as it turns out.)

In game mechanics terms, the undead are the magically reanimated remains of a formerly living creature--usually a bipedal humanoid, but not always. The classic zombies and skeletons are slow, mindless, and follow the orders of the necromancer that animated them (other varieties of undead like ghosts or liches are quite beyond what anyone will be able to deal with in the campaign at the start and will be covered later). Mindless undead that stray out of their necromancer's control radius will become wandering monsters or stand idle waiting for more instructions, depending on many various factors. All corporeal undead can sense the life force of living creatures and are driven to attack anyone alive. Zombies missing their eyes are still capable of tracking living creatures; skeletons (other than this specific one) never had functioning eyes but are still capable of stalking and attacking living beings.

Using the rules for alignments, the creation of a skeleton or a zombie is an evil act. It involves bringing death energy into the world and infusing a dead creature with the desire to kill indiscriminately. Those are not good acts, although I'm sure some necromancers had relatively good intentions when making a squadron of skeletons to defend themselves. And there are some industrial workplaces in the game world (Peachtree and the USA forbid this) where skeletons man textile looms or industrial furnaces, removing the need for the factory owners to pay their laborers; they can also work their undead slaves until they literally fall apart.

Divine spellcasters such as clerics and druids can channel their faith and patron [(G/g)od(dess)(s/es)]' energy into turning undead (which will force them away from the cleric or druid as long as the effect holds) or destroying them outright with sufficient power. Arcane spellcasters generally are not able to do this; it's one of the differences between arcane and divine magic. Evil clerics / druids can be capable of commanding or healing lesser undead instead of turning them.