Hypostases

Hypostases are a Normal Boss group. They are lifeforms that have reached the highest level of elemental purity. Hypostases are invulnerable when their cube-like shells are surrounding them and can only be damaged when their diamond-shaped cores are exposed, typically during or after they perform an attack. Their attacks deal elemental damage of their type and they are immune to said type. After losing 95% of their health, Hypostases will typically enter a revival phase (with the exception of the Pyro Hypostasis) in which it spawns several elemental objects of its type that must be dealt with within a time limit, which if done successfully, will either immediately defeat the Hypostasis or allow it to be defeated the next time it takes lethal DMG.

Adventurer_handbook
Elemental hypostases are life forms which have completely abandoned their former appearance and biological structure, making them able to reach the highest level of elemental purity. They are ultra-compact structures with a high mass, and are the highest forms of elemental structures, usually formed either at a location bursting with elemental energy or at a clogged ley line.
Description
Hypostases are a Normal Boss group. They are lifeforms that have reached the highest level of elemental purity. Hypostases are invulnerable when their cube-like shells are surrounding them and can only be damaged when their diamond-shaped cores are exposed, typically during or after they perform an attack. Their attacks deal elemental damage of their type and they are immune to said type. After losing 95% of their health, Hypostases will typically enter a revival phase (with the exception of the Pyro Hypostasis) in which it spawns several elemental objects of its type that must be dealt with within a time limit, which if done successfully, will either immediately defeat the Hypostasis or allow it to be defeated the next time it takes lethal DMG.