Legendary Resistance (3/Day). If the necromancer fails a saving throw, it may choose to succeed instead. 

Magic Resistance. The necromancer has advantage on saving throws against spells and magical effects. 

Spellcasting. The necromancer is a 9th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). The necromancer has following wizard spells prepared:
Cantrips (at will): chill touch, fire bolt, mage hand, ray of frost 
1st level (4 slots): mage hand, magic missile, ray of sickness, shield 
2nd level (3 slots): hold person, invisibility, misty step 
3rd level (3 slots): animate dead, counterspell, fireball 
4th level (3 slots): blight, sickening radiance
5th level (1 slots): cloudkill 

Turn Resistance. The necromancer has advantage on saving throws against effects that turn undead. 

Undead Fortitude. If damage reduces the necromancer to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the necromancer drops to 1 hit point instead. 

Touch of Undeath. Melee Spell Attack: +7 to hit, reach 5 ft., 1 target. Hit: 33 (6d10) necrotic damage. If this damage reduces a creature to 0 hit points, it dies. If the creature killed is a humanoid, it becomes a zombie at the start of the necromancer's next turn.