Einherjar is an end game event involving 6-36 players facing off against the forces serving the Celestial Avatar of Darkness, Odin. Along with your teammates, you will begin your assault on one of the three initial Chambers located in the Hazhalm Testing Grounds, where you will face off against hordes of smaller monsters and a far more powerful monster belonging to the group known as the Guardian of Asgarth. Repeated victories will allow you to progress through two more tiers of Chambers until finally reaching the fourth, where you will face off against the Lord of the Asgarth himself, Odin.
Besides being rewarded after each Einherjar raid with Ampoules of Therion Ichor, which can be traded in for items and equipment, successfully completing a Chamber will spawn an Armoury Crate that can contain synthesis materials and abjurations for uncursing certain armor pieces.
The minimum requirements for participating in Einherjar are as follows
An item, similar to the Timeless Hourglass for Dynamis, called a Smouldering Lamp must be purchased in Nashmau from the NPC Kilusha for 180,000 gil. In order to reserve an Einherjar Chamber, a player must trade this item to the Entry Gate in Hazhalm Testing Grounds. Once this happens, the Smouldering Lamp will change into a Glowing Lamp which is distributed to the members of the party to be used to enter the Chamber.
Upon entering an Einherjar Chamber, players have 30 minutes to defeat all the monsters in the zone, which consist of one or more groups of weaker monsters, and a single powerful monster belonging to the group known as the Guardian of Asgarth. Clearing out all the monsters in the chamber will cause an Armoury Crate to spawn.
Apart from these monsters, any one of four special notorious monsters will spawn after an unspecified amount of time and will despawn if not defeated quickly.
Winning an Einherjar zone will also reward you with a specific key item (the name of which is a Feather that refers to what chamber that was completed, i.e. Siegrune's Feather) which grants you access to the next tier of Chambers. In order to reach the next Einherjar tier, at least one player in your group must have the key items of all three chambers of the previous tier, while the rest of the group will only need a single one of said key items.
Furthermore, in order to enter Odin's Chamber, at least one player in your group must have the key items of all nine of the previous chambers, while the other players will just need to have one feather from each tier of chambers in order to enter.