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.  

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  While following the command of a captain, the undead gains additional abilities based on the captain's level:   2nd: Turn Resistance. The undead has advantage on saving throws against any effect that turns undead.   6th: Necrotizing Bite. The undead's bite deals an extra 1d6 necrotic damage on a hit.   10th: Regeneration. The undead regains 1 hit point at the start of its turn . If the undead takes fire or radiant damage, this trait doesn't function at the start of the undead's next turn. The undead dies only if it starts its turn with 0 hit points and doesn't regenerate.   14th: Improved Necrotizing Bite. The necrotic damage for the undead's bite increases to 7 (2d6), and the undead regains hit points equal to the necrotic damage dealt on a hit.   18th: Undead Horde. A Humanoid slain by the undead's bite attack rises 24 hours later as a zombie under the captain's control, unless the Humanoid is restored to life or its body is destroyed. The captain can have no more than six zombies under its control at one time. The zombies remain under the captain's control for 24 hours, after which the captain must cast the animate dead spell or use similar magic to reassert control over them.