The helmet of Aegir (also called Aegishjalmur) is a Galdrastafir from Icelandic magic.
This magical symbol is represented in different forms, but all representations are variations of crosses (similar to a compass rose) with a three-pointed end.
It is mentioned in the heroic poetry of the Edda and was used to instill fear and protection against the abuse of power. Likewise, it should make the wearer imperceptible to the eyes of the enemy. During Christianization, wearing the Aegishjalmur was even punished with death.