Undead Fortitude. If damage reduces the zombie pirate 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.

One Shot. For its very first attack the zombie pirate attempts to fire its pistol.  It takes no penalty for making a ranged attack against a target within 5 ft.  Roll a d6.  On a 1 or 2 the pistol backfires and the zombie rolls the damage dice and does the damage to itself, the pistol no longer works.  On a 3 or 4 the pistol jams, doing no damage, and no longer works.  On a 4 or a 5 the pistol fires at the intended target.  To Hit and damage die are rolled as normal.  There are no rounds remaining in the pistol and the zombie pirate doesn't know how to reload it.

Multiattack. The zombie pirate makes a pistol attack and a rapier attack or two rapier attacks.

Rapier. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

Pistol. Ranged Weapon Attack: +4 to hit, range 30/90 ft., one target. Hit: 7 (1d10 + 2) piercing damage.