Crested Helmet (5e Equipment)
From D&D Wiki
A crested helmet or cassis crista, is a heavy, all-enclosing helmet with grill-like visor. It most commonly used by gladiators to provide protection when otherwise unarmored, and acts as a mask for anonymity. This helmet is usually made of bronze and decorated with a plume crest.
You have disadvantage on Wisdom (Perception) checks while wearing a great helm.
Partial armor only grants its AC bonus if you are wearing light or no armor. If you are wearing only one piece of partial armor, you are still considered to be wearing your base armor type (e.g. no armor or light armor). If you are wearing two or three pieces of partial armor, you are considered to be wearing the heaviest type of partial armor worn (e.g. medium or heavy armor). You cannot wear four or more pieces of partial armor.