Magic Resistance. The death knight has advantage on saving throws against spells and other magical effects.

Marshal Undead. Unless the death knight is incapacitated, it and undead creatures of its choice within 60 feet of it have advantage on saving throws against features that turn undead.

Spellcasting. The death knight is a 19th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following paladin spells prepared:

1st level (4 slots): command, compelled duel, searing smite

2nd level (3 slots): hold person, magic weapon

3rd level (3 slots): dispel magic, elemental weapon

4th level (3 slots): banishment, staggering smite

5th level (2 slots): destructive wave (necrotic)

Emberlost is still a creature but it's influence spreads across the dreadnought. 
With it's action it can take one of the following actions:
Control Figurehead. Emberlost can make the figurehead take an action.
Control Body. Emberlost uses his original body allowing it to take an action.
Spellcasting. The Death Knight can use it's spells.

Additionally it can take 3 Crew Actions. Emberlost can take only 2 of these actions if it has fewer than ten crew and only 1 action if it has fewer than three.

Fire Ballistas. Emberlost can fire one of its ballistas.
Fire Mangonels. Emberlost can fire one of its mangonels.
Move. The ship can use its helm to move with its oars or sails. As part of this move, it can use its naval ram.

Hull

Armor Class 16
Hit Points 500 (damage threshold 15)

Control: Helm

Armor Class 18
Hit Points 50
Move up to the speed of one of the ship’s movement components, with one 90-degree turn. If the helm is destroyed, the shipcan’t turn.

Movement: Oars

Armor Class 12
Hit Points 100; -5 ft. speed per 25 damage taken
Speed (water) 30 ft. (requires at least 10 crew)

Movement: Sails

Armor Class 12
Hit Points 100; -10 ft. speed per 25 damage taken
Speed (water) 45 ft.; 15 ft. while sailing into the wind; 60 ft. while sailing with the wind

Movement: Hellfire Orb

The iron brazier mounted on the mainmast contains an ever-burning orb of flame. The spirit of the death knight can see and talk through this orb, perceiving its surroundings with darkvision to a range of 120 feet. Once per day, the death knight can hurl a magical ball of hellfire.

Hellfire Orb (1/Day). Emberlost hurls a magical ball of fire that explodes at a point it can see within 240 feet of it. Each creature in a 20-foot-radius sphere centered on that point must make a DC 18 Dexterity saving throw. The sphere spreads around corners. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Weapons: Ballistas (2)

Armor Class 15
Hit Points 50 each
Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3d10) piercing damage.

Weapons: Mangonels (2)

Armor Class 15
Hit Points 100 each
Ranged Weapon Attack: +5 to hit, range 200/800 ft. (can’t hit targets within 60 ft. of it), one target. Hit: 27 (5d10) bludgeoning damage.

Weapon: Naval Ram

Armor Class 20
Hit Points 100 (damage threshold 10)
The galley has advantage on all saving throws relating to crashing when it crashes into a creature or an object. Any damage it takes from the crash is applied to the naval ram rather than to the ship. These benefits don’t apply if another vessel crashes into the galley. A successful ram does 16d10 damage

Weapon: Skeletal Figurehead

Armor Class 17
Hit Points 115
Multiattack. The skeleton makes three halberd attacks.
Halberd. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 9 (3d10 + 5) slashing damage.

Weapons: Death Knight

Armor Class 20
Longsword. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) slashing damage, or 10 (1d10 + 5) slashing damage if used with two hands, plus 18 (4d8) necrotic damage.