After the defeat of Zepha and Zesha, the party decides to head deeper within the Fortress Island to find the Gurak's Outsider half and shut down the fortress. The party finds the Outsider already damaged by the Gurak, which only amplified the Outsider's rage and power. The Sentinel Beast then appears from the ground and then grabbing Zael and Calista to the surface of the fortress. Zael and Calista are forced to fight Atar on their own until the rest of the party members arrive to the scene. Calista then recites an ancient Arganan story, which gives her the Spirit Attack "Ancient Summon". Using Ancient Summon will allow Calista to summon the Sentinel Beast Mitra which then greatly damages Atar and destroys both its wings, permanently grounding it. Calista comments on how both of the Sentinel Beasts are born the same, yet they are both different; Mitra having been lonely and Atar being enraged.
Then the battle against Atar is very similar to the battle against Mitra. They both have the same weaknesses (Nature Element), both vulnerable to silence, and they both attack similary. However, they do have a few differences. Most of Atar's projectile attacks can inflict poison damage on you and can also create a large Poison Magic Circle for a period of time. Unlike Mitra's laser attack, Atar insteads summons multiple swift lightning bolts on the party. The best way to predict this attack is when Atar's casting number is in purple.
In Co-Op mode, Atar is one of the bosses that can be fought. After half of it's health it gone, Mitra is then summoned to the battle field as another enemy making this battle very tough.