Divine Smite. Upon hitting a creature with a melee weapon attack, Zaixios can expend a spell slot to deal additional radiant damage to the target.  The damage is 1d8, plus 1d8 per level of the slot.  This damage increases by 1d8 if the target is a fiend or an undead.

Divine Health. Zaixios is immune to disease.

Blue Dragon Mask. Zaixios' mask is a legendary magic item with four charges.  He can expend one charge to do any of the following: use the Blue Dragon Breath action, Add 1d6 lightning damage to one attack, prevent the target of an attack from taking reactions until the start of his next turn.

Channel Divinity

Zaixios has one use of Channel Divinity, which can be either of the two options below:

Conquering Presence. You can use your Channel Divinity to exude a terrifying presence. As an action, you force each creature of your choice that you can see within 30 feet of you to make a Wisdom saving throw. On a failed save, a creature becomes frightened of you for 1 minute. The frightened creature can repeat this saving throw at the end of each of its turns, ending the effect on itself on a success.

Guided Strike. You can use your Channel Divinity to strike with supernatural accuracy. When you make an attack roll, you can use your Channel Divinity to gain a +10 bonus to the roll. You make this choice after you see the roll, but before the DM says whether the attack hits or misses.

Spellcasting. Zaixios is a 3rd-level spellcaster. Its spellcasting ability is Charisma (spell save DC 12, +4 to hit with spell attacks). Zaixios has following paladin spells prepared:  
1st level (3 slots): Bless, Compelled Duel, Thunderous Smite, Shield of Faith, Armor of Agathys, Command

Dash. Zaixios doubles his speed for this turn.

Dodge. Until the start of his next turn, Zaixios has advantage on Dexterity saving throws and attacks made against him have disadvantage.

Disengage. Until the end of his turn, Zaixios does not provoke opportunity attacks.

Divine Sense (3/day). Until the end of your next turn, you know the location of any celestial, fiend, or undead within 60 feet of you that is not behind total cover. You know the type (celestial, fiend, or undead) of any being whose presence you sense, but not its identity.

Lay on Hands (15 total healing). Zaixios has a pool of 15 hitpoints that he can bestow any number of on a creature he can touch as an action.

Silver Dragon Breath. Zaixios exhales a 15 ft. cone of frost. Any creatures in the blast must make a DC 12 Constitution saving throw or take 2d6 cold damage.

Blue Dragon Breath (One mask charge). Zaixios exhales lightning in a 30 ft. by 5 ft. line.  Any creatures in this line must make a DC 12 Dexterity saving throw or take 2d6 lightning damage.

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

Opportunity Attack. If a creature exits Zaixios' range without disengaging, Zaixios can use his reaction to make one melee weapon attack against that creature.