Zombie

The Zombie gets back up, and how hard you hit decides how often

How to run it. 15 Hit Points, AC 8, Speed 20 feet, one Slam at +3 for 1d8 + 1. A +5 attacker hits it 90% of the time. On its own it is a speed bump. In a crowd, in a narrow corridor, it is a clock that keeps ticking.

The rule that matters. Undead Fortitude. When damage drops a zombie to 0, it makes a Constitution save against DC 5 plus the damage taken, and on a success it stays up with 1 Hit Point. Radiant damage and Critical Hits skip the save entirely. Its Con save is +3, so the size of the finishing blow is everything:

A longsword (1d8 + 3) blow that drops it leaves it standing about 58% of the time; a greatsword (2d6 + 3) about 45%. There is no natural-20 rescue on a save, so a big enough hit is a sure kill.

The common mistake is rolling the save in silence. Say it out loud: "it takes the hit, drops, and pulls itself up again." The players learn the rule from the fiction, and the cleric's Sacred Flame suddenly becomes the most wanted spell at the table.

For players. Spend your weak attacks first and your biggest hit last. Chip it to low HP with a dagger, then land the heavy blow, because the save counts the whole blow, not just what was left. Radiant damage ends the argument.

The human angle. Zombie stories are rehearsal. In a 2021 study of 310 people during the COVID-19 pandemic, Coltan Scrivner and colleagues found that fans of "prepper" genres, zombie and apocalypse films among them, were both more prepared and more resilient, and horror fans in general were more resilient. A slow shuffling horde at the table is safe practice for the real feeling of "it keeps coming".

Source: Scrivner, C., Johnson, J. A., Kjeldgaard-Christiansen, J., & Clasen, M. (2021). Pandemic practice: Horror fans and morbidly curious individuals are more psychologically resilient during the COVID-19 pandemic. Personality and Individual Differences, 168, 110397. doi:10.1016/j.paid.2020.110397