Necrotic Smite (3/day). After hitting a creature with his longsword, Zaixios can expend one charge of Necrotic Smite to deal an additional 2d8 necrotic damage.

Spellcasting. The [monster name] is a 5th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). Zaixios has following spells prepared:
Cantrips (at will): Green-Flame Blade
1st level (4 slots): Command, Compelled Duel, Searing Smite

2nd level (2 slots): Hold Person, Darkness

Note: Zaixios counts as a 5th level character for the purposes of calculating Green-Flame Blade's damage.

Multiattack. Zaixios makes two attacks with his longsword, then uses his Command Undead feature.

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

Command Undead.  One undead within 30' of Zaixios that he can see uses its reaction to move up to its speed and make a weapon attack against a creature in range.

Zaixios has 3 legendary actions, which he can use at the end of someone else's turn.

Move. Zaixios moves up to his speed.  This does not provoke opportunity attacks.

Command Undead. Zaixios uses his Command Undead feature.

Longsword (costs 3 actions). Zaixios makes one attack with his longsword.