A fitting topic for a dead blog, right?
The undead mostly blow in D&D. This comes from the fact that all the undead tropes are familiar to players, both in style and mechanics. So no one is afraid of the undead and everyone knows how to leverage their strengths and weaknesses. Let's change that up a bit.
The undead have to be shrouded in the unknown in order to invoke any fear in the players. This means much more than just shuffling their abilities around or contriving new strengths and vulnerabilities for them. On a fundamental level, we have to strip the Cleric's ability to trivialize the undead. Then, we have to strip in-game and meta-game knowledge about the undead. From there, we can build undead foes while sprinkling in some of the (ir)rational fears that we share as people.
Let's take up that second item here with a look at zombies in the 17th century. All undead need to have some sort of 'fear aura' that eliminates the 'undead hunter' trope. You know, the expert in killing undead, of which there is invariably one hanging out in each town. Nearly every encounter with the undead should result in death or fear in order to maintain the milieu.