Hulking size. Any being, of smaller size than Lucien, has disadvantage on grappling him

Spellcasting. Lucien of Takar is a spellcaster that can cast spells up to the level 5. Its spellcasting ability is Charisma (spell save DC 18, +8 to hit with spell attacks). Lucien of Takar can cast the following spells that were given to him by Tiamat : Breath Weapon, Vitriolic sphere, Elemental weapon, Absorb element.

Mounted combattant. While Lucien is mounted and isn't incapacitated, he gains the following benefits:

• He has advantage on melee attack rolls against any unmounted creature that is smaller than his mount.
• He can force an attack targeted at his mount to target him instead.
• If his mount is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Regeneration. Lucien regains 10 hit points at the start of his turn.

Multiattack : Lucien of Takar can make two attacks with his halberd, or two attacks with his longsword, or one attack with his longsword and one attack with his shield.

Halberd +2 : Weapon Attack: + 12 to hit, reach 5 ft., one target. Hit: 2d10 + 10 slashing damage plus 2d10 acid damage.

Longsword : Weapon attack: +12 to hit, one target, Hit: 2d8 (2d10 if wielded with two hands) + 10 slashing damage.

Shield : Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 2d4 + 8 bludgeoning damage. If the target is Large or smaller target, it must succeed on a Strength saving throw DC 20 or be knocked prone.

Breath Weapon : As an action, Lucien of Takar can exhale destructive energy. When he uses his breath weapon, each creature in the area of the exhalation must make a Dexterity saving throw. The DC for this saving throw equals 17. A creature takes 9d6 acid damage on a failed save, and half as much damage on a successful one. Distance : 5 by 30 ft. Each turn, roll a d6. On a 4, 5 or 6, Lucien gains the ability to recast this spell.

Elemental weapon (At will, concentration) : As an action, a nonmagical weapon Lucien touch becomes a magic weapon. For the duration, the weapon has a +2 bonus to attack rolls and deals an extra 2d4 acid damage of the chosen type when it hits.

Elemental Bane (1/short rest, concentration) : Lucien chooses one creature he can see within range. The target must succeed on a Constitution saving throw (DC 18) or be affected by the spell for its duration. The first time each turn the affected target takes acid damage, the target takes an extra 3d6 damage of that type. Moreover, the target loses any resistance to that damage type until the spell ends

Vitriolic sphere (3/long rest) : As an action, Lucien can point at a location within 150 feet, and a glowing 1-foot-diameter ball of emerald acid streaks there and explodes in a 20-foot-radius sphere. Each creature in that area must make a Dexterity saving throw. On a failed save, a creature takes 12d4 acid damage and another 5d4 acid damage at the end of its next turn. On a successful save, a creature takes half the initial damage and no damage at the end of its next turn. 

Potion of superior healing : Lucien can use his action to drink one of the two potions of superior healing he has to regain 8d4 + 8 hit points when he drinks one of them

Absorb element (In response to being damaged by acid, cold, fire, lightning or thunder) (3/short rest) : The spell captures some of the incoming energy, lessening its effect on Lucien and storing it for his next melee attack. He has resistance to the triggering damage type until the start of his next turn. Also, the first time he hits with a melee attack on his next turn, the target takes an extra 4d6 damage of the triggering type, and the spell ends. Automatically ends at the end of his next turn.

Legendary Resistance (2/Day) : If Lucien of Takar fails a saving throw, he can choose to succeed instead

Lucien  can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Lucien  regains spent legendary actions at the start of its turn.

Halberd +2 : Weapon Attack: + 12 to hit, reach 5 ft., one target. Hit: 2d10 + 10 slashing damage plus 2d10 acid damage.

Longsword : Weapon attack: +12 to hit, one target, Hit: 2d8 (2d10 if wielded with two hands) + 10 slashing damage.

Shield bash (costs 2 actions) : Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 2d4 + 8 bludgeoning damage. If the target is Large or a smaller target, it must succeed on a Strength saving throw DC 20 or be knocked prone.