Slider (boss)

From Rebirth of the Night Wiki

The Slider is the animated guardian of bronze dungeons in The Aether. Unlike most mobs, this solid construct can't be damaged at all by blades and must be attacked with a pickaxe in order to inflict damage to it. This boss drops its eye, used to create an unattuned focus and Arcane Eye which is one of two items that can be used to enter the Beneath