Relentless
The first time it would drop to 0 HP, it remains at 1 HP; the second time, it falls normally.
Undead Challenge 2
A zombie is a dead body kept moving by dark magic or a curse. It may rise in a graveyard, underground, or at the site of an ancient disaster; its creation often involves someone or something that disturbed the peace of the dead. Decaying equipment, a personal possession, or a familiar face can connect the threat to a settlement's past. A zombie represents both a danger and a person whose story did not end with dignity.
Most zombies have little will of their own. They follow orders, repeat simple actions, or react to the presence of the living according to the nature of their animation. One may wander alone, while a group controlled by a necromancer pursues a shared goal. Their lack of fear and fatigue makes them persistent, but they cannot normally reassess a situation or negotiate as a living person would. A zombie bound to an unusually powerful memory or curse may be an exception.
Relentless means that the first time the zombie would drop to zero HP, it remains at one HP; the second time, it falls normally. It can therefore appear defeated before it actually stops being a threat. Its rules do not automatically give it an infectious bite or require a weakness to blows to the head. The party can separate a group, secure an escape route, or seek the source of its animation. Investigating the past and removing the cause may matter more than destroying more bodies that keep returning.

The first time it would drop to 0 HP, it remains at 1 HP; the second time, it falls normally.