Would you rather fight a beholder or a zombie? This is a much more complicated question than you might realize. Look at this through the eyes of your character and not through the eyes of a meta-gamer. In-character what is the scariest monster you can imagine? For me it’s undead more than any other.
Most monsters are, well, monstrous. They are clearly different than you and they must be destroyed. The beholder is an abomination. It’s scary, and a big party of what makes it scary is that it doesn’t conform to a physical shape you’re comfortable with. It’s a giant floating ball with eyestalks swirling about. Even if you’d never seen a beholder before and knew nothing about it, your initial instinct as an adventurer would be to attack and destroy something so awful.
Other monsters may look familiar or resemble something you know, but their increased size, ferocious attitude or raw destructive power leave little doubt in your mind that the thing before you is a monster. This is certainly the case with dragons. As an adventurer your job is clear – kill it.
Undead, however, fall into a category all their own. The most frightening type of monster is one that looks like someone you know and love. A random zombie shuffling towards you is scary, but now image it’s your mother, your wife or even your child. That’s horrific. Think of the psychological impact that has on a person. You clearly see it coming toward you and your rational brain is saying “kill the monster” but the emotional part of your brain is saying “that’s someone I know and love.” You have doubt, which brings about hesitation. This psychological edge is what makes undead so terrifying.
In the example above I used zombies, which are scary in their own right, but what if the undead creature is something less horrific looking. Take another popular type of undead, the vampire for example. Part of what makes a vampire so deadly is their ability to assume a pleasant looking form. They don’t look dead. They don’t even look undead. Anyone caught unaware is suddenly vulnerably to a potentially horrific assault by said vampire.
Even if you are aware that the creature is a vampire, how would you react if it was a loved one? You know that your brother was attacked and ravaged by a vampire, yet there he is standing right before your very eyes, looking as perfect and healthy as ever. He’s likely even trying to convince you that he managed to survive the attack and is fine. Your rational mind screams “liar!” but your heart wants to believe that your brother is fine. You’ve hoped for the best possibly outcome and now you’re trying to convince yourself it actually happened that way. You want to believe that he’s ok. Even when he moves to attack you, whether it’s a pouncing charge indented to pin you down or it’s a more subtle gesture of embrace like a hug, your mind and emotions will cloud your judgment. You don’t want to fight. You don’t want to kill.
Now let’s look at the beholder again. As soon as it moves towards you, you attack to kill. No doubt, no hesitation. It’s a monster. Your job is to kill it before it kills you or anyone else you love (like your brother the vampire or your mother the zombie – see how much undead can mess with your thinking).
Even lycanthrope, scary villains in their own right, don’t carry the psychological shock of undead. In their normal human form they may indeed be your friend or family member but when they wolf-out and transform into the beast it’s a lot easier to forget that just seconds earlier they were your uncle or your girlfriend. They don’t become truly dangerous until they take on the bestial animal form. When that happens, the veil of doubt is removed and you have no trouble seeing the threat for what it is. Any personal ties you might have felt are more easily broken when you see the lycanthrope in its animal form. No chance of lulling you into a false sense of security when they’ve just become a giant wolf.
It’s this psychological impact that give undead the edge over all other monsters in my opinion. No matter how seasoned an adventurer you are, no matter how many beholders you’ve killed or werewolves you’ve slain, you’re going to second-guess yourself when you fight undead. When you recognize the face of the creature before you, your instincts will betray you. The realization that yesterday it as your wife and today it’s a zombie trying to kill reminds you of your own vulnerability.
The challenge with D&D or other role playing games is capturing the sense of dread undead bring rather than focusing on just the raw mechanics of combat. Beholders and dragons are generally tougher in terms of levels and hit points than zombies and vampires, but the undead are much more likely to mess with a character’s mind. It’s up to the DM to really try and covey this sense of fear and uncertainty when running undead. The players should likewise try to see these encounters through the eyes of their characters and not just as another opportunity to kill a monster and earn XP.
By focusing on the role-playing more than the number crunching, encounters against undead can easily be more memorable, and likely more difficult, than any battle against a beholder or even a dragon.