Explode. When the mage is reduced to 0 hit points, it explodes in a cloud of ash. Any creature within 5 feet of it must succeed on a DC 14 Constitution saving throw or take 11 (2d10) necrotic damage.
Spider Climb. The mage can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Unusual Nature. The mage doesn't require air or drink.
Spellcasting. The mage casts one of the following spells, using Charisma as the spellcasting ability (spell save DC 13):
1/day: darkness, dimension door, fly, hypnotic pattern
Multiattack. The mage makes two Ray of Cold attacks.
Energy Drain. Melee or Ranged Spell Attack: +5 to hit, reach 5 ft. or range 30 ft., one creature. Hit:22 (4d10) necrotic damage. A Humanoid reduced to 0 hit points by this attack dies and instantly transforms into a free-willed shadow under the DM's control.
Ray of Cold. Ranged Spell Attack: +5 to hit, range 120 ft., one target. Hit:11 (2d8 + 2) cold damage.
A ship of vampirates needs a spellcaster to operate the spelljamming helm. A vampirate mage rarely, if ever, leaves the helm.