Reckless.  At the start of its turn, the undead can gain advantage on all melee weapon attack rolls during that turn, but attack rolls against it have advantage until the start of its next turn.

Undead Fortitude.  If damage reduces the undead 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 undead drops to 1 hit point instead.

Greataxe.  Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (1d12 + 3) slashing damage.