Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Sidekick:

Martial Role

1st-level Warrior feature

Defender. The sidekick can use its reaction to impose disadvantage on the attack roll of a creature within 5 feet of it whose target isn’t the sidekick, provided the sidekick can see the attacker.

Second Wind

2nd-level Warrior feature

The sidekick can use a bonus action on its turn to regain hit points equal to 1d10 + its level in this class. Once it uses this feature, it must finish a short or long rest before it can use it again.

The sidekick can use this feature twice between rests starting at 20th level.

Improved Critical

3rd-level Warrior feature

The sidekick’s attack rolls score a critical hit on a roll of 19 or 20 on the d20.

Ability Score Improvement

4th-level Warrior feature

At 4th level and again at 8th, 12th, 14th, 16th, and 19th level, the sidekick increases one ability score of your choice by 2, or the sidekick increases two ability scores of your choice by 1. The sidekick can’t increase an ability score above 20 using this feature.

Extra Attack

6th-level Warrior feature

The sidekick can attack twice, instead of once, whenever it takes the Attack action on its turn.

The number of attacks increases to three when the sidekick reaches 15th level.

If the sidekick has the Multiattack action, it can use Extra Attack or Multiattack on a turn, not both.

Attack. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 4 (1d8 + 4) bludgeoning damage.

Dodge.

Second Wind

2nd-level Warrior feature

The sidekick can use a bonus action on its turn to regain hit points equal to 1d10 + its level in this class. Once it uses this feature, it must finish a short or long rest before it can use it again.

Defender. The sidekick can use its reaction to impose disadvantage on the attack roll of a creature within 5 feet of it whose target isn’t the sidekick, provided the sidekick can see the attacker.