Geomancers are very strange and terrifying illagers that only spawn in the deepslate layer of the caves. They summon magical pillars to trap and explode you, which can be quite a dangerous combo.
They drop amethyst shards and amethyst dust.
They also drop the “Crystalline” armor trim 1/8th of the time if the armor trim addon is installed.
Look at those tattered clothes and ratty beard… he’s been down in the caves a while.