Cordana Felsong

TLDR: If you just came to this page to find out what to do because Cordana has an immunity shield on: There is an invisible mob in one of the corners of the platform called Avatar of Vengeance. The person with Elune's light needs to make this mob visible and then the group needs to kill it to make her shield drop.

Cordana Felsong is one of the most complex bosses of the Initial Dungeons released with Legion. She is the final boss of Vault of the Wardens

It is very important that the person who is holding [spell=201359] knows what they are doing.

When Cordana casts [spell=197422] she will float up into the air. She will start summoning rows of spirits that do a lot of damage to any player who touches them, however the row of spirits is invisible. In this row of spirits there will be one small gap to pass through safely. The person with Elune's Light can reveal the spirits by going near them with the light (or throwing it). I find the easiest thing to do on this phase is watch when they spawn. When the rows of spirits spawn they will be visible to everyone for a second. Instead of running around trying to reveal them it is a good idea for the person with the light to watch for this and then throw the light where the gap is, using it as a marker so that the whole party knows where to go.

Periodically throughout the fight Cordana will cast [spell=213583] on the ground, which is a circular patch of badness that will slowly grow bigger until it fills the entire platform dealing damage to anyone that touches it. The person with Elune's Light can shrink this patch into nothingness by running near it with the Light or throwing the Light at it.

At some point in the fight Cordana will spawn an Avatar of Vengeance. While this Avatar is up Cordana will have a green shield that protects her from damage. The Avatar of vengeance will be invisible in one of the corners of the platform near one of the owl statues. The person with Elune's light will need to run to the corner with the Light, or throw the Light at the corner to reveal the Avatar of Vengeance. Once the Avatar is revealed it will need to be killed. Please note that the Tank should NOT do this. The Tank should stay with Cordana. If the Tank tries to help and takes Cordana over to the corner of the platform they are likely to get kicked off the side when she casts Turn Kick.

An easy way to find which corner the Avatar of Vengeance is standing in is to watch to see where the green glaives are being thrown from. The Avatar should be killed quickly. If it is not killed quickly Cordana will spawn another one and if that is not killed quickly, another.

This boss pops a big, green bubble to shield herself. You cannot attack her again until you find the hidden mob (Avatar of Vengeance) in one of the corners. Just follow the green spells that spin from one corner toward the middle of the room. One person in the group should have [spell=201359] and can cast it or run toward that corner to reveal the Avatar. Burn it down quickly as a group, then go back to the boss.

Luckily hunters can use Flare to expose the hidden mob if the person with Elune's Light is spaced out or can't seem to follow the green spells that give away its location. ;)

A couple hints if your doing this solo:

Change the text window to combat log. So you can see if your doing no damage and you should be alerted to go find the shadow thing.

Another is to raid mark her to keep an eye out for which one she is.