A necromancer, also known as a summoner, is a powerful undead hostile mob found in Minecraft Dungeons that can summon undead mobs. They look similar to the Nameless One but are slightly smaller, about 2.5 blocks tall, and wear a purple robe with blue markings.

Appearance:

A necromancer is a large skeleton that wears blue turfs, a cape with blue stripes near the end, a golden belt with a blue gem in the middle, has greenish eyes, and wears a gold crown-like headwear. Necromancers always hold a staff with a blue orb placed upon a stick.

Basic attack:

Necromancers use their staff to summon zombies and armored zombies to attack heroes. They summon mobs much more often than the Nameless One. Should the hero get too close, the necromancer immediately flees until they are a safe distance from the hero, and then summon mobs again.

Strong attack:

Necromancers also rapidly fire damaging laser orbs from their staff to heroes. The laser orbs deal less damage than the zombie's melee attack but more damage than the skeleton's ranged attack, and they also knock the hero back.

https://minecraft.fandom.com/wiki/Minecraft_Dungeons:Necromancer